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| author | H. Peter Anvin <hpa@zytor.com> | 2009-06-07 15:16:24 -0700 |
|---|---|---|
| committer | H. Peter Anvin <hpa@zytor.com> | 2009-06-07 15:16:24 -0700 |
| commit | b68960be5d1f10b3936492c93557c34e46cdebef (patch) | |
| tree | b63bebea77a04319a5b982c0e7e4f61c40b00ef3 | |
| parent | f05ef9550d95c4ea386bd1879e0e03e81fd0a951 (diff) | |
| download | syslinux.git-b68960be5d1f10b3936492c93557c34e46cdebef.tar.gz syslinux.git-b68960be5d1f10b3936492c93557c34e46cdebef.tar.xz syslinux.git-b68960be5d1f10b3936492c93557c34e46cdebef.zip | |
memdisk: remove the requirement that CS_BASE < 64K
Remove the requirement that CS_BASE is below 64K, based on its use in
the A20 test code. This means it can now be located anywhere in high
memory, but it is still a constant.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
| -rw-r--r-- | memdisk/memdisk16.asm | 35 |
1 files changed, 25 insertions, 10 deletions
diff --git a/memdisk/memdisk16.asm b/memdisk/memdisk16.asm index b21c8b80..1d77bf94 100644 --- a/memdisk/memdisk16.asm +++ b/memdisk/memdisk16.asm @@ -204,7 +204,6 @@ err_a20: db 'ERROR: A20 gate not responding!',13,10,0 alignb 4 SavedSSSP resd 1 ; Place to save SS:SP Return resd 1 ; Return value -A20Test resw 1 ; Space to test A20 A20Tries resb 1 section .data @@ -383,22 +382,38 @@ a20_done: popad ; This routine tests if A20 is enabled (ZF = 0). This routine ; must not destroy any register contents. ; + +; This is the INT 1Fh vector, which is standard PCs is used by the +; BIOS when the screen is in graphics mode. Even if it is, it points to +; data, not code, so it should be safe enough to fiddle with. +A20Test equ (1Fh*4) + a20_test: + push ds push es push cx - push ax - mov cx,0FFFFh ; HMA = segment 0FFFFh - mov es,cx + push eax + xor ax,ax + mov ds,ax ; DS == 0 + dec ax + mov es,ax ; ES == 0FFFFh mov cx,32 ; Loop count - mov ax,[A20Test] -.a20_wait: inc ax - mov [A20Test],ax - io_delay ; Serialize, and fix delay - cmp ax,[es:A20Test+CS_BASE+10h] + mov eax,[A20Test] + cmp eax,[es:A20Test+10h] + jne .a20_done + push eax +.a20_wait: + inc eax + mov [A20Test],eax + io_delay + cmp eax,[es:A20Test+10h] loopz .a20_wait -.a20_done: pop ax + pop dword [A20Test] ; Restore original value +.a20_done: + pop eax pop cx pop es + pop ds ret disable_a20: |