85 lines
3.9 KiB
Plaintext
85 lines
3.9 KiB
Plaintext
# Fill a region of memory with zeroes.
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== code
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# instruction effective address register displacement immediate
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# . op subop mod rm32 base index scale r32
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# . 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
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zero-out: # start : address, len : int
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# pseudocode:
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# curr/ESI = start
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# i/ECX = 0
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# while true
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# if (i >= len) break
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# *curr = 0
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# ++curr
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# ++i
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#
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# . prolog
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55/push-EBP
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89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
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# . save registers
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50/push-EAX
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51/push-ECX
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52/push-EDX
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56/push-ESI
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# curr/ESI = start
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8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 6/r32/ESI 8/disp8 . # copy *(EBP+8) to ESI
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# i/ECX = 0
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31/xor 3/mod/direct 1/rm32/ECX . . . 1/r32/ECX . . # clear ECX
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# EDX = len
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8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 2/r32/EDX 0xc/disp8 . # copy *(EBP+12) to EDX
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$zero-out:loop:
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# if (i >= len) break
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39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX
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7d/jump-if-greater-or-equal $zero-out:end/disp8
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# *curr = 0
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c6 0/subop/copy 0/mod/direct 6/rm32/ESI . . . . . 0/imm8 # copy byte to *ESI
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# ++curr
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46/increment-ESI
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# ++i
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41/increment-ECX
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eb/jump $zero-out:loop/disp8
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$zero-out:end:
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# . restore registers
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5e/pop-to-ESI
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5a/pop-to-EDX
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59/pop-to-ECX
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58/pop-to-EAX
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# . epilog
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89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
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5d/pop-to-EBP
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c3/return
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test-zero-out:
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# . prolog
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55/push-EBP
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89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
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# region/ECX = 34, 35, 36, 37
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68/push 0x37363534/imm32
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89/copy 3/mod/direct 1/rm32/ECX . . . 4/r32/ESP . . # copy ESP to ECX
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# zero-out(ECX, 3)
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# . . push args
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68/push 3/imm32/len
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51/push-ECX
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# . . call
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e8/call zero-out/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
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# first 3 bytes cleared, fourth left alone
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# . check-ints-equal(*ECX, 0x37000000, msg)
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# . . push args
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68/push "F - test-zero-out"/imm32
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68/push 0x37000000/imm32
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ff 6/subop/push 0/mod/indirect 1/rm32/ECX . . . . . . # push *ECX
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# . . call
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e8/call check-ints-equal/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
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# . epilog
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89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
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5d/pop-to-EBP
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c3/return
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# . . vim:nowrap:textwidth=0
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