Merge pull request #20 from akkartik/charles-l-print-int-decimal

exercise: reimplement print-int-decimal

1 files changed, 97 insertions, 85 deletions

diff --git a/subx/074print-int-decimal.subx b/subx/074print-int-decimal.subx
index 86015a00..e5dbaa30 100644
--- a/subx/074print-int-decimal.subx
+++ b/subx/074print-int-decimal.subx
@@ -6,38 +6,32 @@
 # . 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
 
 #? Entry:  # run a single test, while debugging
-#?     e8/call test-print-int32-decimal/disp32
+#?     e8/call test-print-int32-decimal-negative/disp32
+#?
 #?     # syscall(exit, Num-test-failures)
 #?     8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/EBX   Num-test-failures/disp32          # copy *Num-test-failures to EBX
 #?     b8/copy-to-EAX  1/imm32/exit
 #?     cd/syscall  0x80/imm8
 
 print-int32-decimal:  # out : (address stream), n : int32
-    # works by generating characters from lowest to highest and pushing them
-    # to the stack, before popping them one by one into the stream
-    #
-    # pseudocode:
-    #   copy ESP to EBX
-    #   EAX = |n|
-    #   while true
-    #     if (EAX == 0) break
-    #     sign-extend EAX into EDX
-    #     EAX, EDX = EAX/10, EAX%10
-    #     push EDX
-    #   if n < 0
-    #     push '-' - '0' = -3
-    #   max/ECX = &out->data[out->length]
-    #   w/EAX = out->write
-    #   curr/EDI = &out->data[out->write]
-    #   while true
-    #     if (ESP == EBX) break
-    #     if (curr >= max) abort
-    #     pop into EDX
-    #     EDX += '0'  # convert decimal digit to ascii
-    #     *curr = DL
-    #     ++curr
-    #     ++w
-    #   out->write = w
+    # PSEUDOCODE
+    # based on K&R itoa: https://en.wikibooks.org/wiki/C_Programming/stdlib.h/itoa
+    # push SENTINAL
+    # if n >= 0:
+    #   goto digit-encode
+    # else:
+    #   n = -n
+    #   negative = true
+    # while true:
+    #   push n % 10 + '0'
+    #   n = n / 10
+    #   break if n == 0
+    # if negative:
+    #   push '-'
+    # while true:
+    #   EAX = pop
+    #   break if EAX == SENTINAL
+    #   out->data[out->write++] = EAX
     #
     # . prolog
     55/push-EBP
@@ -48,70 +42,88 @@ print-int32-decimal:  # out : (address stream), n : int32
     52/push-EDX
     53/push-EBX
     57/push-EDI
-    # copy ESP to EBX
-    89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBX
-    # ten/ECX = 10
-    b9/copy-to-ECX  0xa/imm32
-    # EAX = |n|
-    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           0/r32/EAX   0xc/disp8       .                 # copy *(EBP+12) to EAX
-    3d/compare-EAX-with  0/imm32
-    7d/jump-if-greater-or-equal $print-int32-decimal:read-loop/disp8
-$print-int32-decimal:negative:
-    f7          3/subop/negate      3/mod/direct    0/rm32/EAX    .           .             .           .           .               .                 # negate EAX
-$print-int32-decimal:read-loop:
-    # if (EAX == 0) break
+
+    # negative = false
+    bf/copy-to-EDI  0/imm32                                                                                                                           # EDI = 0
+
+    # push sentinal for $append-loop
+    68/push-sentinel   0/imm32
+
+    # EAX = n
+    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           0/r32/EAX   0xc/disp8       .                 # EAX = *(EBP+12)
+    # if n >= 0 jump to digit-encode
     3d/compare-EAX-and  0/imm32
-    74/jump-if-equal $print-int32-decimal:read-break/disp8
-    # sign-extend
+    7d/jump-if-greater-eq $digit-encode:prelude/disp8
+    # else
+    # n = -n
+    f7  3/subop/negate              3/mod/direct    0/rm32/EAX                                                                                        # EAX = -EAX
+
+    # negative = true
+    bf/copy-to-EDI  1/imm32                                                                                                                           # EDI = 1
+
+$digit-encode:prelude:
+    # EBX = n
+    8b/copy                         3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # EBX = EAX
+
+$digit-encode-loop:
+    # EAX = EBX(n)
+    89/copy                         3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # EAX = EBX
+    b9/copy-to-ECX    0xa/imm32
     99/sign-extend-EAX-into-EDX
-    # EAX, EDX = divide-with-remainder EAX/ten, EAX%ten
-    f7          7/subop/divide-by   3/mod/direct    1/rm32/ECX    .           .             .           .           .               .                 # divide EDX:EAX by ECX, storing quotient in EAX and remainder in EDX
-    # push EDX
+    # EDX:EAX(n) / ECX(10)
+
+    # EDX, EAX = n % 10, n / 10
+    f7          7/subop/idiv        3/mod/direct    1/rm32/ECX    .           .             .           .           .               .                 # divide EDX:EAX by ECX, storing quotient in EAX and remainder in EDX
+    81          0/subop/add         3/mod/direct    2/rm32/EDX    .           .             .           .           .               0x30/imm32        # EDX += '0' (0x30)
+
+    # push n % 10 + '0'
     52/push-EDX
-    eb/jump  $print-int32-decimal:read-loop/disp8
-$print-int32-decimal:read-break:
-    # if (n < 0) push('-')
-    81          7/subop/compare     1/mod/*+disp8   5/rm32/EBP    .           .             .           .           0xc/disp8       0/imm32           # compare *(EBP+12)
-    7d/jump-if-greater-or-equal $print-int32-decimal:write/disp8
-$print-int32-decimal:push-negative:
-    68/push  -3/imm32/dash-minus-zero
-    # fall through
-$print-int32-decimal:write:
-    # EDI = out
-    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
-    # max/ECX = &out->data[out->length]
-    8b/copy                         1/mod/*+disp8   7/rm32/EDI    .           .             .           1/r32/ECX   8/disp8         .                 # copy *(EDI+8) to ECX
-    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/EDI  1/index/ECX   .           1/r32/ECX   0xc/disp8       .                 # copy EDI+ECX+12 to ECX
-    # w/EAX = out->write
-    8b/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy *EDI to EAX
-    # curr/EDI = &out->data[out->write]
-    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    7/base/EDI  0/index/EAX   .           7/r32/EDI   0xc/disp8       .                 # copy EDI+EAX+12 to EDI
-$print-int32-decimal:write-loop:
-    # if (ESP == EBX) break
-    39/compare                      3/mod/direct    4/rm32/ESP    .           .             .           3/r32/EBX   .               .                 # compare ESP and EBX
-    74/jump-if-equal  $print-int32-decimal:write-break/disp8
-    # if (curr >= max) abort
-    39/compare                      3/mod/direct    7/rm32/EDI    .           .             .           1/r32/ECX   .               .                 # compare EDI and ECX
-    7d/jump-if-greater-or-equal  $print-int32-decimal:abort/disp8
-    # pop into EDX
-    5a/pop-into-EDX
-    # EDX += '0'
-    81          0/subop/add         3/mod/direct    2/rm32/EDX    .           .             .           .           .               0x30/imm32/zero   # add to EDX
-$print-int32-decimal:write-char:
-    # *curr = DL
-    88/copy-byte                    0/mod/indirect  7/rm32/EDI    .           .             .           2/r32/DL    .               .                 # copy DL to byte at *ECX
-    # ++curr
-    47/increment-EDI
-    # ++w
-    40/increment-EAX
-    eb/jump  $print-int32-decimal:write-loop/disp8
-$print-int32-decimal:write-break:
-    # out->write = w
-    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
-    89/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy EAX to *EDI
+
+    # EBX = n / 10
+    89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # EBX = EAX
+
+    # loop if n / 10 > 0
+    3d/compare-EAX-and  0/imm32
+    7f/jump-if-greater $digit-encode-loop/disp8
+    # else:
+
+    8b/copy                         3/mod/direct    7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # EAX = EDX
+    # . if !negative: goto append-loop
+    3d/compare-EAX-and 0/imm32
+    74/jump-if-equal   $append-loop:prelude/disp8
+    # . else
+    68/push-negative   0x2d/imm32 # push '-'
+
+$append-loop:prelude:
+    # EBX = &out
+    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           3/r32/EBX   8/disp8         .                 # copy *(EBP+8) to EBX
+
+$append-loop:
+    # EAX = next_character
+    58/pop-to-EAX
+
+    3d/compare-EAX                  .               .             .           .             .           .           .               0/imm32           # EAX == 0
+    74/jump-if-equal   $append-loop:break/disp8
+
+    # ECX = out->write
+    8b/copy                         0/mod/indirect  3/rm32/EBX    .           .             .           1/r32/ECX   .               .                 # ECX = *EBX
+
+    # out->data[out->write] = next_character
+    # . ECX = &out->data[out->write]
+    8d/copy-address                 1/mod/*+disp8   4/rm32/sib    3/base/EBX  1/index/ECX   .           1/r32/ECX   0xc/disp8       .                 # copy EBX+ECX+12 to ECX
+    # . out->data[out->write] = EAX
+    89/copy                         0/mod/indirect  1/rm32/ECX    .           .             .           0/r32/ECX  .                .                 # *ECX = EAX
+
+    # out->write++
+    ff          0/subop/increment   0/mod/indirect  3/rm32/EBX    .           .             .           .           .               .                 # (*EBX)++
+    eb/jump $append-loop/disp8
+
+$append-loop:break:
+
 $print-int32-decimal:end:
+    # . reclaim locals
     # . restore registers
-    5f/pop-to-EDI
+    57/pop-to-EDI
     5b/pop-to-EBX
     5a/pop-to-EDX
     59/pop-to-ECX