4901

1 files changed, 63 insertions, 0 deletions

diff --git a/subx/053new-segment.subx b/subx/053new-segment.subx
new file mode 100644
index 00000000..0aee0539
--- /dev/null
+++ b/subx/053new-segment.subx
@@ -0,0 +1,63 @@
+# Create a new segment (for data) using mmap().
+
+== 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
+
+# main:  (manual test if this is the last file loaded)
+    # EAX = new-segment(0x1000)
+    # . . push args
+    68/push  0x1000/imm32
+    # . . call
+    e8/call  new-segment/disp32
+    # . . discard args
+    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
+
+    # write to *EAX to check that we have access to the newly-allocated segment
+    c7          0/subop/copy        0/mod/direct    0/rm32/EAX    .           .             .           .           .               0x34/imm32        # copy to *EAX
+
+    # syscall(exit, EAX)
+    89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
+    b8/copy-to-EAX  1/imm32/exit
+    cd/syscall  0x80/imm8
+
+new-segment:  # len : int -> address
+    # . prolog
+    55/push-EBP
+    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
+    53/push-EBX
+    # copy len to _mmap-new-segment->len
+    # TODO: compute _mmap-new-segment+4 before runtime
+    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
+    bb/copy-to-EBX  _mmap-new-segment/imm32
+    89/copy                         1/mod/*+disp8   3/rm32/EBX    .           .             .           0/r32/EAX   4/disp8         .                 # copy EAX to *(EBX+4)
+    # mmap(_mmap-new-segment)
+    bb/copy-to-EBX  _mmap-new-segment/imm32
+    b8/copy-to-EAX  0x5a/imm32/mmap
+    cd/syscall  0x80/imm8
+$new-segment:end:
+    # . epilog
+    5b/pop-to-EBX
+    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
+    5d/pop-to-EBP
+    c3/return
+
+== data
+
+# various constants used here were found in the Linux sources (search for file mman-common.h)
+_mmap-new-segment:  # type mmap_arg_struct
+    # addr
+    00 00 00 00  # null
+    # len
+    00 00 00 00  # 0x1000
+    # protection flags
+    03 00 00 00  # PROT_READ | PROT_WRITE
+    # sharing flags
+    22 00 00 00  # MAP_PRIVATE | MAP_ANONYMOUS
+    # fd
+    ff ff ff ff  # -1 since MAP_ANONYMOUS is specified
+    # offset
+    00 00 00 00  # 0 since MAP_ANONYMOUS is specified
+
+# . . vim:nowrap:textwidth=0