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>

diff --git a/memdisk/memdisk16.asm b/memdisk/memdisk16.asm
index b21c8b8..1d77bf9 100644 (file)
--- 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: