diff options
Diffstat (limited to 'subx/076zero-out.subx')
| -rw-r--r-- | subx/076zero-out.subx | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/subx/076zero-out.subx b/subx/076zero-out.subx new file mode 100644 index 00000000..bc19dc21 --- /dev/null +++ b/subx/076zero-out.subx @@ -0,0 +1,84 @@ +# Fill a region of memory with zeroes. + +== 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 + +zero-out: # start : address, len : int + # pseudocode: + # curr/ESI = start + # i/ECX = 0 + # while true + # if (i >= len) break + # *curr = 0 + # ++curr + # ++i + # + # . prolog + 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 + 56/push-ESI + # curr/ESI = start + 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 6/r32/ESI 8/disp8 . # copy *(EBP+8) to ESI + # i/ECX = 0 + 31/xor 3/mod/direct 1/rm32/ECX . . . 1/r32/ECX . . # clear ECX + # EDX = len + 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 2/r32/EDX 0xc/disp8 . # copy *(EBP+12) to EDX +$zero-out:loop: + # if (i >= len) break + 39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX + 7d/jump-if-greater-or-equal $zero-out:end/disp8 + # *curr = 0 + c6 0/subop/copy 0/mod/direct 6/rm32/ESI . . . . . 0/imm8 # copy byte to *ESI + # ++curr + 46/increment-ESI + # ++i + 41/increment-ECX + eb/jump $zero-out:loop/disp8 +$zero-out:end: + # . restore registers + 5e/pop-to-ESI + 5a/pop-to-EDX + 59/pop-to-ECX + 58/pop-to-EAX + # . epilog + 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP + 5d/pop-to-EBP + c3/return + +test-zero-out: + # . prolog + 55/push-EBP + 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP + # region/ECX = 34, 35, 36, 37 + 68/push 0x37363534/imm32 + 89/copy 3/mod/direct 1/rm32/ECX . . . 4/r32/ESP . . # copy ESP to ECX + # zero-out(ECX, 3) + # . . push args + 68/push 3/imm32/len + 51/push-ECX + # . . call + e8/call zero-out/disp32 + # . . discard args + 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP + # first 3 bytes cleared, fourth left alone + # . check-ints-equal(*ECX, 0x37000000, msg) + # . . push args + 68/push "F - test-zero-out"/imm32 + 68/push 0x37000000/imm32 + ff 6/subop/push 0/mod/indirect 1/rm32/ECX . . . . . . # push *ECX + # . . call + e8/call check-ints-equal/disp32 + # . . discard args + 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP + # . epilog + 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 |