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# 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
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