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James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
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49-05.md
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@ -29,52 +29,54 @@ chapters.
**LISTING 49.6 L49-6.ASM**
; Shows the page at the specified offset in the bitmap. Page is displayed when
; this routine returns.
; C near-callable as: void ShowPage(unsigned int StartOffset);
INPUT_STATUS_1 equ 03dah ;Input Status 1 register
CRTC_INDEX equ 03d4h ;CRT Controller Index reg
START_ADDRESS_HIGH equ 0ch ;bitmap start address high byte
START_ADDRESS_LOW equ 0dh ;bitmap start address low byte
```nasm
; Shows the page at the specified offset in the bitmap. Page is displayed when
; this routine returns.
; C near-callable as: void ShowPage(unsigned int StartOffset);
INPUT_STATUS_1 equ 03dah ;Input Status 1 register
CRTC_INDEX equ 03d4h ;CRT Controller Index reg
START_ADDRESS_HIGH equ 0ch ;bitmap start address high byte
START_ADDRESS_LOW equ 0dh ;bitmap start address low byte
ShowPageParms struc
dw 2 dup (?) ;pushed BP and return address
StartOffset dw ? ;offset in bitmap of page to display
ShowPageParms ends
.model small
.code
public _ShowPage
_ShowPage proc near
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
; Wait for display enable to be active (status is active low), to be
; sure both halves of the start address will take in the same frame.
mov bl,START_ADDRESS_LOW ;preload for fastest
mov bh,byte ptr StartOffset[bp] ; flipping once display
mov cl,START_ADDRESS_HIGH ; enable is detected
mov ch,byte ptr StartOffset+1[bp]
mov dx,INPUT_STATUS_1
WaitDE:
in al,dx
test al,01h
jnz WaitDE ;display enable is active low (0 = active)
; Set the start offset in display memory of the page to display.
mov dx,CRTC_INDEX
mov ax,bx
out dx,ax ;start address low
mov ax,cx
out dx,ax ;start address high
; Now wait for vertical sync, so the other page will be invisible when
; we start drawing to it.
mov dx,INPUT_STATUS_1
WaitVS:
in al,dx
test al,08h
jz WaitVS ;vertical sync is active high (1 = active)
pop bp ;restore caller's stack frame
ret
_ShowPage endp
end
ShowPageParms struc
dw 2 dup (?) ;pushed BP and return address
StartOffset dw ? ;offset in bitmap of page to display
ShowPageParms ends
.model small
.code
public _ShowPage
_ShowPage proc near
push bp ;preserve caller's stack frame
mov bp,sp ;point to local stack frame
; Wait for display enable to be active (status is active low), to be
; sure both halves of the start address will take in the same frame.
mov bl,START_ADDRESS_LOW ;preload for fastest
mov bh,byte ptr StartOffset[bp] ; flipping once display
mov cl,START_ADDRESS_HIGH ; enable is detected
mov ch,byte ptr StartOffset+1[bp]
mov dx,INPUT_STATUS_1
WaitDE:
in al,dx
test al,01h
jnz WaitDE ;display enable is active low (0 = active)
; Set the start offset in display memory of the page to display.
mov dx,CRTC_INDEX
mov ax,bx
out dx,ax ;start address low
mov ax,cx
out dx,ax ;start address high
; Now wait for vertical sync, so the other page will be invisible when
; we start drawing to it.
mov dx,INPUT_STATUS_1
WaitVS:
in al,dx
test al,08h
jz WaitVS ;vertical sync is active high (1 = active)
pop bp ;restore caller's stack frame
ret
_ShowPage endp
end
```
### Works Fast, Looks Great {#Heading8}