133 lines
3.8 KiB
NASM
133 lines
3.8 KiB
NASM
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;*******************************************************************************
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;memset.asm - set a section of memory to all one byte
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;
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; Copyright (c) Microsoft Corporation. All rights reserved.
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;
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;Purpose:
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; contains the memset() routine
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;
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;*******************************************************************************
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.code
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;***
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;char *memset(dst, value, count) - sets "count" bytes at "dst" to "value"
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;
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;Purpose:
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; Sets the first "count" bytes of the memory starting
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; at "dst" to the character value "value".
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;
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; Algorithm:
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; char *
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; memset (dst, value, count)
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; char *dst;
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; char value;
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; unsigned int count;
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; {
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; char *start = dst;
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;
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; while (count--)
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; *dst++ = value;
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; return(start);
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; }
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;
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;Entry:
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; char *dst - pointer to memory to fill with value
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; char value - value to put in dst bytes
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; int count - number of bytes of dst to fill
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;
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;Exit:
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; returns dst, with filled bytes
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;
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;Uses:
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;
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;Exceptions:
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;
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;*******************************************************************************
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public memset
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memset proc
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;
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; On entry: rcx=dest, rdx=value, r8=count
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;
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test r8,r8 ; 0?
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jz short toend ; if so, nothing to do
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xor rax,rax
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mov al,dl ; al = value
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mov rdx,r8 ; rdx = "count"
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mov r8,rcx ; scroll away dest for return
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; Align address on qword boundary
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push rdi ; preserve rdi
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mov rdi,rcx ; rdi = dest pointer
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cmp rdx,8 ; if it's less then 8 bytes
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jb tail ; tail needs rdi and rdx to be initialized
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neg rcx
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and rcx,7 ; ecx = # bytes before dword boundary
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jz short qwords ; jump if address already aligned
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sub rdx,rcx ; edx = adjusted count (for later)
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adjust_loop:
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mov [rdi],al
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add rdi,1
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sub rcx,1
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jnz adjust_loop
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qwords:
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; set all 8 bytes of eax to [value]
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mov rcx,rax ; rcx=0/0/0/0/0/0/0/value
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shl rax,8 ; rax=0/0/0/0/0/0/value/0
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add rax,rcx ; rax=0/0/0/0/0/0/0/val/val
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mov rcx,rax ; rcx=0/0/0/0/0/0/0/val/val
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shl rax,10h ; rax=0/0/0/0/val/val/0/0
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add rax,rcx ; rax = all 4 bytes = [value]
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mov rcx,rax ; rcx = 0/0/0/0/val/val/val/val
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shl rax,20h ; rax = val/val/val/val/0/0/0/0/
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add rax,rcx ; rax = val/val/val/val/val/val/val/val
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; Set qword-sized blocks
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mov rcx,rdx ; move original count to ecx
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and rdx,7 ; prepare in edx byte count (for tail loop)
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shr rcx,3 ; adjust ecx to be dword count
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jz tail ; jump if it was less then 4 bytes
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rep stosq
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main_loop_tail:
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test rdx,rdx ; if there is no tail bytes,
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jz finish ; we finish, and it's time to leave
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; Set remaining bytes
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tail:
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mov [rdi],al ; set remaining bytes
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add rdi,1
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sub rdx,1 ; if there is some more bytes
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jnz tail ; continue to fill them
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; Done
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finish:
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mov rax,r8 ; return dest pointer
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pop rdi ; restore edi
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ret
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toend:
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mov rax,rcx ; return dest pointer
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ret
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memset endp
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end
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