7782 - baremetal: provide a trace stream

3 files changed, 868 insertions, 84 deletions

diff --git a/baremetal/107trace.subx b/baremetal/107trace.subx
new file mode 100644
index 00000000..36c993eb
--- /dev/null
+++ b/baremetal/107trace.subx
@@ -0,0 +1,866 @@
+# primitives for emitting traces to 'Trace-stream', and for tests to make assertions on its contents
+#
+# Trace-stream looks like a regular stream, but it has its own dedicated region
+# of memory, currently [0x02000000, 0x03000000):
+#   write: int  # index at which writes go
+#   read: int  # index that we've read until
+#   data: (array byte)  # prefixed by size as usual
+#
+# primitives for operating on Trace-stream (arguments in quotes):
+#   - initialize-trace-stream: clear stream contents
+#   - trace: adds a 'line' to Trace-stream
+#   - check-trace-contains: scans from Trace-stream's start for a matching 'line', prints a 'message' to stderr on failure
+#   - check-trace-scans-to: scans from Trace-stream's read pointer for a matching 'line', prints a 'message' to stderr on failure
+
+== code
+#   instruction                     effective address                                                   register    displacement    immediate
+# . op          subop               mod             rm32          base        index         scale       r32
+# . 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes
+
+# Append a string to the given trace stream.
+# Silently give up if it's already full. Or truncate the string if there isn't enough room.
+trace:  # line: (addr array byte)
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    56/push-esi
+    57/push-edi
+    # var edi: (addr stream byte) = Trace-stream
+    bf/copy-to-edi 0x02000000/imm32/Trace-stream
+    # esi = line
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         6/r32/esi   8/disp8         .                 # copy *(ebp+8) to esi
+    # var ecx: int = t->write
+    8b/copy                         0/mod/indirect  7/rm32/edi    .           .             .           1/r32/ecx   .               .                 # copy *edi to ecx
+    # var edx: int = t->size
+    8b/copy                         1/mod/*+disp8   7/rm32/edi    .           .             .           2/r32/edx   8/disp8         .                 # copy *(edi+8) to edx
+    # eax = _append-3(&t->data[t->write], &t->data[t->size], line)
+    # . . push line
+    56/push-esi
+    # . . push &t->data[t->size]
+    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/edi  2/index/edx   .           3/r32/ebx   0xc/disp8       .                 # copy edi+edx+12 to ebx
+    53/push-ebx
+    # . . push &t->data[t->write]
+    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/edi  1/index/ecx   .           3/r32/ebx   0xc/disp8       .                 # copy edi+ecx+12 to ebx
+    53/push-ebx
+    # . . call
+    e8/call  _append-3/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # if (eax == 0) return
+    3d/compare-eax-and  0/imm32
+    74/jump-if-=  $trace:end/disp8
+    # t->write += eax
+    01/add                          0/mod/indirect  7/rm32/edi    .           .             .           0/r32/eax   .               .                 # add eax to *edi
+    # refresh ecx = t->write
+    8b/copy                         0/mod/indirect  7/rm32/edi    .           .             .           1/r32/ecx   .               .                 # copy *edi to ecx
+    # eax = _append-3(&t->data[t->write], &t->data[t->size], line)
+    # . . push line
+    68/push  Newline/imm32
+    # . . push &t->data[t->size]
+    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/edi  2/index/edx   .           3/r32/ebx   0xc/disp8       .                 # copy edi+edx+12 to ebx
+    53/push-ebx
+    # . . push &t->data[t->write]
+    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/edi  1/index/ecx   .           3/r32/ebx   0xc/disp8       .                 # copy edi+ecx+12 to ebx
+    53/push-ebx
+    # . . call
+    e8/call  _append-3/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # t->write += eax
+    01/add                          0/mod/indirect  7/rm32/edi    .           .             .           0/r32/eax   .               .                 # add eax to *edi
+$trace:end:
+    # . restore registers
+    5f/pop-to-edi
+    5e/pop-to-esi
+    5b/pop-to-ebx
+    5a/pop-to-edx
+    59/pop-to-ecx
+    58/pop-to-eax
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+test-trace-single:
+    # clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # trace("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(*Trace-stream->data, 41/A 62/b 0a/newline 00, msg)
+    # . . push args
+    68/push  "F - test-trace-single"/imm32
+    68/push  0x0a6241/imm32/Ab-newline
+    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           0x0200000c/disp32                 # push *(Trace-stream+12)
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # end
+    c3/return
+
+test-trace-appends:
+    # clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # trace("C")
+    # . . push args
+    68/push  "C"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # trace("D")
+    # . . push args
+    68/push  "D"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(*Trace-stream->data, 43/C 0a/newline 44/D 0a/newline, msg)
+    # . . push args
+    68/push  "F - test-trace-appends"/imm32
+    68/push  0x0a440a43/imm32/C-newline-D-newline
+    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           0x0200000c/disp32                 # push *(Trace-stream+12)
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # end
+    c3/return
+
+test-trace-empty-line:
+    # clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # trace("")
+    # . . push args
+    68/push  ""/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(*Trace-stream->write, 0, msg)
+    # . . push args
+    68/push  "F - test-trace-empty-line"/imm32
+    68/push  0/imm32
+    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           0x02000000/disp32                 # push *Trace-stream
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # end
+    c3/return
+
+check-trace-contains:  # line: (addr string), msg: (addr string)
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # rewind-stream(Trace-stream)
+    # . . push args
+    68/push  0x02000000/imm32/Trace-stream
+    # . . call
+    e8/call  rewind-stream/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-trace-scans-to(line, msg)
+    # . . push args
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
+    # . . call
+    e8/call  check-trace-scans-to/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+$check-trace-contains:end:
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+check-trace-scans-to:  # line: (addr string), msg: (addr string)
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    50/push-eax
+    # eax = trace-scan(line)
+    # . . push args
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
+    # . . call
+    e8/call  trace-scan/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(eax, 1, msg)
+    # . . push args
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
+    68/push  1/imm32
+    50/push-eax
+    # . . call
+    e8/call check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+$check-trace-scans-to:end:
+    # . restore registers
+    58/pop-to-eax
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+# Start scanning from Trace-stream->read for 'line'. If found, update Trace-stream->read and return true.
+trace-scan:  # line: (addr array byte) -> result/eax: boolean
+    # pseudocode:
+    #   push Trace-stream->read
+    #   while true:
+    #     if Trace-stream->read >= Trace-stream->write
+    #       break
+    #     if next-line-matches?(Trace-stream, line)
+    #       skip-next-line(Trace-stream)
+    #       dump saved copy of Trace-stream->read
+    #       return true
+    #     skip-next-line(Trace-stream)
+    #   pop saved copy of Trace-stream->read
+    #   return false
+    #
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    51/push-ecx
+    56/push-esi
+    # esi = Trace-stream
+    be/copy-to-esi  0x02000000/imm32/Trace-stream
+    # ecx = Trace-stream->write
+    8b/copy                         0/mod/indirect  6/rm32/esi    .           .             .           1/r32/ecx                   .                 # copy *esi to ecx
+    # push Trace-stream->read
+    ff          6/subop/push        1/mod/*+disp8   6/rm32/esi    .           .             .           .           4/disp8         .                 # push *(esi+4)
+$trace-scan:loop:
+    # if (Trace-stream->read >= Trace-stream->write) return false
+    39/compare                      1/mod/*+disp8   6/rm32/esi    .           .             .           1/r32/ecx   4/disp8         .                 # compare ecx with *(esi+4)
+    7d/jump-if->=  $trace-scan:false/disp8
+    # eax = next-line-matches?(Trace-stream, line)
+    # . . push args
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
+    56/push-esi
+    # . . call
+    e8/call  next-line-matches?/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+    # if (eax == false) continue
+    3d/compare-eax-and  0/imm32/false
+    74/jump-if-=  $trace-scan:continue/disp8
+$trace-scan:true:
+    # skip-next-line(Trace-stream)
+    # . . push args
+    56/push-esi
+    # . . call
+    e8/call  skip-next-line/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # dump saved copy of Trace-stream->read
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # return true
+    b8/copy-to-eax  1/imm32/true
+    eb/jump  $trace-scan:end/disp8
+$trace-scan:continue:
+    # skip-next-line(Trace-stream)
+    # . . push args
+    56/push-esi
+    # . . call
+    e8/call  skip-next-line/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    eb/jump  $trace-scan:loop/disp8
+$trace-scan:false:
+    # restore saved copy of Trace-stream->read
+    8f          0/subop/pop         1/mod/*+disp8   6/rm32/esi    .           .             .           .           4/disp8         .                 # pop to *(esi+4)
+    # return false
+    b8/copy-to-eax  0/imm32/false
+$trace-scan:end:
+    # . restore registers
+    59/pop-to-ecx
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+test-trace-scan-first:
+    # setup
+    # . clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # . trace("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # eax = trace-scan("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace-scan/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(eax, 1, msg)
+    # . . push args
+    68/push  "F - test-trace-scan-first"/imm32
+    68/push  1/imm32
+    50/push-eax
+    # . . call
+    e8/call check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # . end
+    c3/return
+
+test-trace-scan-skips-lines-until-found:
+    # setup
+    # . clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # . trace("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # . trace("cd")
+    # . . push args
+    68/push  "cd"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # eax = trace-scan("cd")
+    # . . push args
+    68/push  "cd"/imm32
+    # . . call
+    e8/call  trace-scan/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(eax, 1, msg)
+    # . . push args
+    68/push  "F - test-trace-scan-skips-lines-until-found"/imm32
+    68/push  1/imm32
+    50/push-eax
+    # . . call
+    e8/call check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # . end
+    c3/return
+
+test-trace-second-scan-starts-where-first-left-off:
+    # setup
+    # . clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # . trace("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # . eax = trace-scan("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace-scan/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # second scan fails
+    # . eax = trace-scan("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace-scan/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # check-ints-equal(eax, 0, msg)
+    # . . push args
+    68/push  "F - test-trace-second-scan-starts-where-first-left-off"/imm32
+    68/push  0/imm32
+    50/push-eax
+    # . . call
+    e8/call check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # . end
+    c3/return
+
+test-trace-scan-failure-leaves-read-index-untouched:
+    # setup
+    # . clear-trace-stream()
+    e8/call  clear-trace-stream/disp32
+    # . trace("Ab")
+    # . . push args
+    68/push  "Ab"/imm32
+    # . . call
+    e8/call  trace/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # . check-ints-equal(Trace-stream->read, 0, msg)
+    # . . push args
+    68/push  "F - test-trace-scan-failure-leaves-read-index-untouched/precondition-failure"/imm32
+    68/push  0/imm32
+    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           0x02000004/disp32                 # push *(Trace-stream+4)
+    # . . call
+    e8/call check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # perform a failing scan
+    # . eax = trace-scan("Ax")
+    # . . push args
+    68/push  "Ax"/imm32
+    # . . call
+    e8/call  trace-scan/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # no change in read index
+    # . check-ints-equal(Trace-stream->read, 0, msg)
+    # . . push args
+    68/push  "F - test-trace-scan-failure-leaves-read-index-untouched"/imm32
+    68/push  0/imm32
+    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           0x02000004/disp32                 # push *(Trace-stream+4)
+    # . . call
+    e8/call check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    # . end
+    c3/return
+
+next-line-matches?:  # t: (addr stream byte), line: (addr array byte) -> result/eax: boolean
+    # pseudocode:
+    #   while true:
+    #     if (currl >= maxl) break
+    #     if (currt >= maxt) return false
+    #     if (*currt != *currl) return false
+    #     ++currt
+    #     ++currl
+    #   return *currt == '\n'
+    #
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    56/push-esi
+    57/push-edi
+    # edx = line
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         2/r32/edx   0xc/disp8       .                 # copy *(ebp+12) to edx
+    # var currl/esi: (addr byte) = line->data
+    # . esi = line/edx->data
+    8d/copy-address                 1/mod/*+disp8   2/rm32/edx    .           .             .           6/r32/esi   4/disp8         .                 # copy edx+4 to esi
+    # var maxl/ecx: (addr byte) = &line->data[line->size]
+    # . eax = line/edx->size
+    8b/copy                         0/mod/indirect  2/rm32/edx    .           .                         0/r32/eax   .               .                 # copy *edx to eax
+    # . maxl = &line->data[line->size]
+    8d/copy-address                 0/mod/indirect  4/rm32/sib    6/base/esi  0/index/eax   .           1/r32/ecx   .               .                 # copy edx+eax to ecx
+    # edi = t
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         7/r32/edi   8/disp8         .                 # copy *(ebp+8) to edi
+    # var ebx: (addr byte) = t->data
+    8d/copy-address                 1/mod/*+disp8   7/rm32/edi    .           .             .           3/r32/ebx   0xc/disp8       .                 # copy edi+12 to ebx
+    # var maxt/edx: (addr byte) = &t->data[t->write]
+    # . eax = t->write
+    8b/copy                         0/mod/indirect  7/rm32/edi    .           .                         0/r32/eax   .               .                 # copy *edi to eax
+    # . maxt = &t->data[t->write]
+    8d/copy-address                 0/mod/indirect  4/rm32/sib    3/base/ebx  0/index/eax   .           2/r32/edx   .               .                 # copy ebx+eax to edx
+    # var currt/edi: (addr byte) = &t->data[t->read]
+    # . eax = t/edi->read
+    8b/copy                         1/mod/*+disp8   7/rm32/edi    .           .                         0/r32/eax   4/disp8         .                 # copy *(edi+4) to eax
+    # . currt = &t->data[t->read]
+    8d/copy-address                 0/mod/indirect  4/rm32/sib    3/base/ebx  0/index/eax   .           7/r32/edi   .               .                 # copy ebx+eax to edi
+$next-line-matches?:loop:
+    # if (currl >= maxl) break
+    39/compare                      3/mod/direct    6/rm32/esi    .           .             .           1/r32/ecx   .               .                 # compare esi and ecx
+    73/jump-if-addr>=  $next-line-matches?:break/disp8
+    # if (currt >= maxt) return false
+    # . eax = false
+    b8/copy-to-eax  0/imm32/false
+    39/compare                      3/mod/direct    7/rm32/edi    .           .             .           2/r32/edx   .               .                 # compare edi and edx
+    73/jump-if-addr>=  $next-line-matches?:end/disp8
+    # if (*currt != *currl) return false
+    31/xor                          3/mod/direct    0/rm32/eax    .           .             .           0/r32/eax   .               .                 # clear eax
+    31/xor                          3/mod/direct    3/rm32/eax    .           .             .           3/r32/eax   .               .                 # clear ebx
+    # . eax: byte = *currt
+    8a/copy-byte                    0/mod/indirect  7/rm32/edi    .           .                         0/r32/eax   .               .                 # copy *edi to eax
+    # . ebx: byte = *currl
+    8a/copy-byte                    0/mod/indirect  6/rm32/esi    .           .                         3/r32/ebx   .               .                 # copy *esi to ebx
+    # . eax >= ebx
+    39/compare                      3/mod/direct    0/rm32/eax    .           .             .           3/r32/ebx   .               .                 # compare eax and ebx
+    # . eax = false
+    b8/copy-to-eax  0/imm32/false
+    75/jump-if-!=  $next-line-matches?:end/disp8
+    # ++currt
+    47/increment-edi
+    # ++currl
+    46/increment-esi
+    eb/jump  $next-line-matches?:loop/disp8
+$next-line-matches?:break:
+    # return *currt == '\n'
+    31/xor                          3/mod/direct    0/rm32/eax    .           .             .           0/r32/eax   .               .                 # clear eax
+    # . eax: byte = *currt
+    8a/copy-byte                    0/mod/indirect  7/rm32/edi    .           .                         0/r32/eax   .               .                 # copy *edi to eax
+    3d/compare-eax-and  0xa/imm32/newline
+    # . eax = false
+    b8/copy-to-eax  1/imm32/true
+    74/jump-if-=  $next-line-matches?:end/disp8
+    b8/copy-to-eax  0/imm32/true
+$next-line-matches?:end:
+    # . restore registers
+    5f/pop-to-edi
+    5e/pop-to-esi
+    5b/pop-to-ebx
+    5a/pop-to-edx
+    59/pop-to-ecx
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+test-next-line-matches?-no-match-1:
+    # next line of "ABABA" does not match "blah blah"
+    # . eax = next-line-matches?(_test-stream-line-ABABA, "blah blah")
+    # . . push args
+    68/push  "blah blah"/imm32
+    68/push  _test-stream-line-ABABA/imm32
+    # . . call
+    e8/call  next-line-matches?/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+    # . check-ints-equal(eax, 0, msg)
+    # . . push args
+    68/push  "F - test-next-line-matches?-no-match-1"/imm32
+    68/push  0/imm32
+    50/push-eax
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    c3/return
+
+test-next-line-matches?-no-match-2:
+    # next line of "ABABA" does not match ""
+    # . eax = next-line-matches?(_test-stream-line-ABABA, "")
+    # . . push args
+    68/push  ""/imm32
+    68/push  _test-stream-line-ABABA/imm32
+    # . . call
+    e8/call  next-line-matches?/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+    # . check-ints-equal(eax, 0, msg)
+    # . . push args
+    68/push  "F - test-next-line-matches?-no-match-2"/imm32
+    68/push  0/imm32
+    50/push-eax
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    c3/return
+
+test-next-line-matches?-no-match-3:
+    # next line of "ABABA" does not match  "AA"
+    # . eax = next-line-matches?(_test-stream-line-ABABA, "AA")
+    # . . push args
+    68/push  "AA"/imm32
+    68/push  _test-stream-line-ABABA/imm32
+    # . . call
+    e8/call  next-line-matches?/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+    # . check-ints-equal(eax, 0, msg)
+    # . . push args
+    68/push  "F - test-next-line-matches?-no-match-3"/imm32
+    68/push  0/imm32
+    50/push-eax
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    c3/return
+
+test-next-line-matches?-match:
+    # next line of "ABABA" matches "ABABA"
+    # . eax = next-line-matches?(_test-stream-line-ABABA, "ABABA")
+    # . . push args
+    68/push  "ABABA"/imm32
+    68/push  _test-stream-line-ABABA/imm32
+    # . . call
+    e8/call  next-line-matches?/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+    # . check-ints-equal(eax, 1, msg)
+    # . . push args
+    68/push  "F - test-next-line-matches?-match"/imm32
+    68/push  1/imm32
+    50/push-eax
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    c3/return
+
+# move t->read to _after_ next newline
+skip-next-line:  # t: (addr stream byte)
+    # pseudocode:
+    #   max = &t->data[t->write]
+    #   i = t->read
+    #   curr = &t->data[t->read]
+    #   while true
+    #     if (curr >= max) break
+    #     ++i
+    #     if (*curr == '\n') break
+    #     ++curr
+    #   t->read = i
+    #
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    # ecx = t
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         1/r32/ecx   8/disp8         .                 # copy *(ebp+8) to ecx
+    # edx = t->data
+    8d/copy-address                 1/mod/*+disp8   1/rm32/ecx    .           .             .           2/r32/edx   0xc/disp8       .                 # copy ecx+12 to edx
+    # eax = t->write
+    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
+    # var max/ebx: (addr byte) = &t->data[t->write]
+    8d/copy-address                 0/mod/indirect  4/rm32/sib    2/base/edx  0/index/eax   .           3/r32/ebx   .               .                 # copy edx+eax to ebx
+    # eax = t->read
+    8b/copy                         1/mod/*+disp8   1/rm32/ecx    .           .             .           0/r32/eax   4/disp8         .                 # copy *(ecx+4) to edx
+    # var curr/ecx: (addr byte) = &t->data[t->read]
+    8d/copy-address                 0/mod/indirect  4/rm32/sib    2/base/edx  0/index/eax   .           1/r32/ecx   .               .                 # copy edx+eax to ecx
+    # var i/edx: int = t->read
+    89/copy                         3/mod/direct    2/rm32/edx    .           .             .           0/r32/eax   .               .                 # copy eax to edx
+$skip-next-line:loop:
+    # if (curr >= max) break
+    39/compare                      3/mod/direct    1/rm32/ecx    .           .             .           3/r32/ebx   .               .                 # compare ecx and ebx
+    73/jump-if-addr>=  $skip-next-line:end/disp8
+    # ++i
+    42/increment-edx
+    # if (*curr == '\n') break
+    31/xor                          3/mod/direct    0/rm32/eax    .           .             .           0/r32/eax   .               .                 # clear eax
+    8a/copy-byte                    0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
+    3d/compare-eax-and  0a/imm32/newline
+    74/jump-if-=  $skip-next-line:end/disp8
+    # ++curr
+    41/increment-ecx
+    # loop
+    eb/jump  $skip-next-line:loop/disp8
+$skip-next-line:end:
+    # ecx = t
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         1/r32/ecx   8/disp8         .                 # copy *(ebp+8) to ecx
+    # t->read = i
+    89/copy                         1/mod/*+disp8   1/rm32/ecx    .           .             .           2/r32/edx   4/disp8         .                 # copy edx to *(ecx+4)
+    # . restore registers
+    5b/pop-to-ebx
+    5a/pop-to-edx
+    59/pop-to-ecx
+    58/pop-to-eax
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+test-skip-next-line-empty:
+    # skipping next line in empty stream leaves read pointer at 0
+    # . skip-next-line(_test-stream-empty)
+    # . . push args
+    68/push  _test-stream-empty/imm32
+    # . . call
+    e8/call  skip-next-line/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # . check-ints-equal(_test-stream-empty->read, 0, msg)
+    # . . push args
+    68/push  "F - test-skip-next-line-empty"/imm32
+    68/push  0/imm32
+    b8/copy-to-eax  _test-stream-empty/imm32
+    8b/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           0/r32/eax   4/disp8         .                 # copy *(eax+4) to eax
+    50/push-eax
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    c3/return
+
+test-skip-next-line-filled:
+    # skipping next line increments read pointer by length of line + 1 (for newline)
+    # . skip-next-line(_test-stream-filled)
+    # . . push args
+    68/push  _test-stream-filled/imm32
+    # . . call
+    e8/call  skip-next-line/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+    # . check-ints-equal(_test-stream-filled->read, 5, msg)
+    # . . push args
+    68/push  "F - test-skip-next-line-filled"/imm32
+    68/push  5/imm32
+    b8/copy-to-eax  _test-stream-filled/imm32
+    8b/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           0/r32/eax   4/disp8         .                 # copy *(eax+4) to eax
+    50/push-eax
+    # . . call
+    e8/call  check-ints-equal/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+    c3/return
+
+# We won't bother to clear the stream contents here. They're just text, never
+# interpreted as addresses.
+clear-trace-stream:
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # Trace-stream->write = 0
+    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           0x02000000/disp32  0/imm32        # copy to *Trace-stream
+    # Trace-stream->read = 0
+    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           0x02000004/disp32  0/imm32        # copy to *(Trace-stream+4)
+    # Trace-stream->size = 0x01000000 - 12
+    c7          0/subop/copy        0/mod/indirect  5/rm32/.disp32            .             .           .           0x02000008/disp32  0x00fffff4/imm32  # copy to *(Trace-stream+8)
+$clear-trace-stream:end:
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+# - helpers
+
+# 3-argument variant of _append
+_append-3:  # out: (addr byte), outend: (addr byte), s: (addr array byte) -> num_bytes_appended/eax
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    51/push-ecx
+    # eax = _append-4(out, outend, &s->data[0], &s->data[s->size])
+    # . . push &s->data[s->size]
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         0/r32/eax   0x10/disp8      .                 # copy *(ebp+16) to eax
+    8b/copy                         0/mod/indirect  0/rm32/eax    .           .             .           1/r32/ecx   .               .                 # copy *eax to ecx
+    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    0/base/eax  1/index/ecx   .           1/r32/ecx   4/disp8         .                 # copy eax+ecx+4 to ecx
+    51/push-ecx
+    # . . push &s->data[0]
+    8d/copy-address                 1/mod/*+disp8   0/rm32/eax    .           .             .           1/r32/ecx   4/disp8         .                 # copy eax+4 to ecx
+    51/push-ecx
+    # . . push outend
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
+    # . . push out
+    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
+    # . . call
+    e8/call  _append-4/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x10/imm32        # add to esp
+$_append-3:end:
+    # . restore registers
+    59/pop-to-ecx
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+# 4-argument variant of _append
+_append-4:  # out: (addr byte), outend: (addr byte), in: (addr byte), inend: (addr byte) -> num_bytes_appended/eax: int
+    # . prologue
+    55/push-ebp
+    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+    # . save registers
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    56/push-esi
+    57/push-edi
+    # num_bytes_appended = 0
+    b8/copy-to-eax  0/imm32
+    # edi = out
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           7/r32/edi   8/disp8         .                 # copy *(ebp+8) to edi
+    # edx = outend
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           2/r32/edx   0xc/disp8       .                 # copy *(ebp+12) to edx
+    # esi = in
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           6/r32/esi   0x10/disp8      .                 # copy *(ebp+16) to esi
+    # ecx = inend
+    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0x14/disp8      .                 # copy *(ebp+20) to ecx
+$_append-4:loop:
+    # if (in >= inend) break
+    39/compare                      3/mod/direct    6/rm32/esi    .           .             .           1/r32/ecx   .               .                 # compare esi with ecx
+    73/jump-if-addr>=  $_append-4:end/disp8
+    # if (out >= outend) abort  # just to catch test failures fast
+    39/compare                      3/mod/direct    7/rm32/edi    .           .             .           2/r32/edx   .               .                 # compare edi with edx
+    73/jump-if-addr>=  $_append-4:end/disp8  # TODO: abort
+    # *out = *in
+    8a/copy-byte                    0/mod/indirect  6/rm32/esi    .           .             .           3/r32/BL    .               .                 # copy byte at *esi to BL
+    88/copy-byte                    0/mod/indirect  7/rm32/edi    .           .             .           3/r32/BL    .               .                 # copy byte at BL to *edi
+    # ++num_bytes_appended
+    40/increment-eax
+    # ++in
+    46/increment-esi
+    # ++out
+    47/increment-edi
+    eb/jump  $_append-4:loop/disp8
+$_append-4:end:
+    # . restore registers
+    5f/pop-to-edi
+    5e/pop-to-esi
+    5b/pop-to-ebx
+    5a/pop-to-edx
+    59/pop-to-ecx
+    # . epilogue
+    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+    5d/pop-to-ebp
+    c3/return
+
+== data
+
+# length-prefixed string containing just a single newline
+# convenient to have when printing messages and so on
+Newline:  # (array byte)
+    # size: int
+    1/imm32
+    # data
+    0a/newline
+
+_test-stream-line-ABABA:  # (stream byte)
+    # write
+    8/imm32
+    # read
+    0/imm32
+    # size
+    8/imm32
+    # data
+    41 42 41 42 41 0a 00 00  # 8 bytes
+
+_test-stream-empty:  # (stream byte)
+    # write
+    0/imm32
+    # read
+    0/imm32
+    # size
+    8/imm32
+    # data
+    00 00 00 00 00 00 00 00  # 8 bytes
+
+_test-stream-filled:  # (stream byte)
+    # write
+    8/imm32
+    # read
+    0/imm32
+    # size
+    8/imm32
+    # data
+    41 41 41 41 0a 41 41 41  # 8 bytes
+
+# . . vim:nowrap:textwidth=0
diff --git a/baremetal/108write.subx b/baremetal/108write.subx
index 43e9bf9b..d94c259f 100644
--- a/baremetal/108write.subx
+++ b/baremetal/108write.subx
@@ -133,88 +133,4 @@ _test-stream:  # (stream byte)
     00 00 00 00 00 00 00 00
     00 00 00 00 00 00 00 00
 
-== code
-
-# 3-argument variant of _append
-_append-3:  # out: (addr byte), outend: (addr byte), s: (addr array byte) -> num_bytes_appended/eax
-    # . prologue
-    55/push-ebp
-    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
-    # . save registers
-    51/push-ecx
-    # eax = _append-4(out, outend, &s->data[0], &s->data[s->size])
-    # . . push &s->data[s->size]
-    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         0/r32/eax   0x10/disp8      .                 # copy *(ebp+16) to eax
-    8b/copy                         0/mod/indirect  0/rm32/eax    .           .             .           1/r32/ecx   .               .                 # copy *eax to ecx
-    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    0/base/eax  1/index/ecx   .           1/r32/ecx   4/disp8         .                 # copy eax+ecx+4 to ecx
-    51/push-ecx
-    # . . push &s->data[0]
-    8d/copy-address                 1/mod/*+disp8   0/rm32/eax    .           .             .           1/r32/ecx   4/disp8         .                 # copy eax+4 to ecx
-    51/push-ecx
-    # . . push outend
-    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
-    # . . push out
-    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
-    # . . call
-    e8/call  _append-4/disp32
-    # . . discard args
-    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0x10/imm32        # add to esp
-$_append-3:end:
-    # . restore registers
-    59/pop-to-ecx
-    # . epilogue
-    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
-    5d/pop-to-ebp
-    c3/return
-
-# 4-argument variant of _append
-_append-4:  # out: (addr byte), outend: (addr byte), in: (addr byte), inend: (addr byte) -> num_bytes_appended/eax: int
-    # . prologue
-    55/push-ebp
-    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
-    # . save registers
-    51/push-ecx
-    52/push-edx
-    53/push-ebx
-    56/push-esi
-    57/push-edi
-    # num_bytes_appended = 0
-    b8/copy-to-eax  0/imm32
-    # edi = out
-    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           7/r32/edi   8/disp8         .                 # copy *(ebp+8) to edi
-    # edx = outend
-    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           2/r32/edx   0xc/disp8       .                 # copy *(ebp+12) to edx
-    # esi = in
-    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           6/r32/esi   0x10/disp8      .                 # copy *(ebp+16) to esi
-    # ecx = inend
-    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0x14/disp8      .                 # copy *(ebp+20) to ecx
-$_append-4:loop:
-    # if (in >= inend) break
-    39/compare                      3/mod/direct    6/rm32/esi    .           .             .           1/r32/ecx   .               .                 # compare esi with ecx
-    73/jump-if-addr>=  $_append-4:end/disp8
-    # if (out >= outend) abort  # just to catch test failures fast
-    39/compare                      3/mod/direct    7/rm32/edi    .           .             .           2/r32/edx   .               .                 # compare edi with edx
-    73/jump-if-addr>=  $_append-4:end/disp8  # TODO: abort
-    # *out = *in
-    8a/copy-byte                    0/mod/indirect  6/rm32/esi    .           .             .           3/r32/BL    .               .                 # copy byte at *esi to BL
-    88/copy-byte                    0/mod/indirect  7/rm32/edi    .           .             .           3/r32/BL    .               .                 # copy byte at BL to *edi
-    # ++num_bytes_appended
-    40/increment-eax
-    # ++in
-    46/increment-esi
-    # ++out
-    47/increment-edi
-    eb/jump  $_append-4:loop/disp8
-$_append-4:end:
-    # . restore registers
-    5f/pop-to-edi
-    5e/pop-to-esi
-    5b/pop-to-ebx
-    5a/pop-to-edx
-    59/pop-to-ecx
-    # . epilogue
-    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
-    5d/pop-to-ebp
-    c3/return
-
 # . . vim:nowrap:textwidth=0
diff --git a/baremetal/boot.hex b/baremetal/boot.hex
index 6a14017d..109fe830 100644
--- a/baremetal/boot.hex
+++ b/baremetal/boot.hex
@@ -70,6 +70,8 @@
 #     see below
 #   heap: [0x01000000, 0x02000000)
 #     see baremetal/120allocate.subx
+#   trace: [0x02000000, 0x03000000)
+#     see baremetal/107trace.subx
 # Consult https://wiki.osdev.org/Memory_Map_(x86) before modifying any of this.
 
 ## 16-bit entry point