source: xtideuniversalbios/trunk/Assembly_Library/Src/Display/CgaSnow.asm@ 415

; Project name  :   Assembly Library
; Description   :   Functions for preventing CGA snow.

;
; XTIDE Universal BIOS and Associated Tools 
; Copyright (C) 2009-2010 by Tomi Tilli, 2011-2012 by XTIDE Universal BIOS Team.
;
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation; either version 2 of the License, or
; (at your option) any later version.
; 
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
; GNU General Public License for more details.      
; Visit http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
;       

; Section containing code
SECTION .text

;--------------------------------------------------------------------
; CgaSnow_IsCgaPresent
;   Parameters:
;       DS:     BDA segment (zero)
;   Returns:
;       CF:     Set if CGA detected
;               Cleared if CGA not detected
;   Corrupts registers:
;       AX
;--------------------------------------------------------------------
ALIGN DISPLAY_JUMP_ALIGN
CgaSnow_IsCgaPresent:
    cmp     WORD [BDA.wVidPort], CGA_STATUS_REGISTER - OFFSET_TO_CGA_STATUS_REGISTER
    jne     SHORT .CgaNotFound

    ; All standard CGA modes use 25 rows but only EGA and later store it to BDA.
    cmp     BYTE [BDA.bVidRows], 25
    jge     SHORT .CgaNotFound
    stc
    ret
ALIGN DISPLAY_JUMP_ALIGN
.CgaNotFound:
    clc
    ret


; CGA snow prevention must be kept optional to avoid unnecessary
; overhead when building programs meant for non-CGA systems.
%ifdef ELIMINATE_CGA_SNOW

;--------------------------------------------------------------------
; CgaSnow_Stosb
; CgaSnow_Stosw
;   Parameters:
;       AL:     Character to output
;       AH:     Attribute to output (CgaSnow_StoswWithoutCgaSnow only)
;       DS:     BDA segment (zero)
;       ES:DI:  Ptr to video memory where to output
;   Returns:
;       DI:     Incremented for next character
;   Corrupts registers:
;       AX, DX
;--------------------------------------------------------------------
ALIGN DISPLAY_JUMP_ALIGN
CgaSnow_Stosb:
    call    LoadCgaStatusRegisterAddressToDXifCgaPresent
    jz      SHORT .StosbWithoutWaitSinceUnknownPort

    mov     ah, al
    cli             ; Interrupt request would mess up timing
    WAIT_UNTIL_SAFE_CGA_WRITE
    mov     al, ah
.StosbWithoutWaitSinceUnknownPort:
    stosb
    sti
    ret

ALIGN DISPLAY_JUMP_ALIGN
CgaSnow_Stosw:
    push    bx
    call    LoadCgaStatusRegisterAddressToDXifCgaPresent
    jz      SHORT .StoswWithoutWaitSinceUnknownPort

    xchg    bx, ax
    cli             ; Interrupt request would mess up timing
    WAIT_UNTIL_SAFE_CGA_WRITE
    xchg    ax, bx
.StoswWithoutWaitSinceUnknownPort:
    stosw
    sti
    pop     bx
    ret


;--------------------------------------------------------------------
; CgaSnow_RepMovsb
;   Parameters:
;       CX:     Number of characters to copy
;       DS:     BDA segment (zero)
;       ES:SI:  Ptr to video memory where to read from
;       ES:DI:  Ptr to video memory where to write to
;   Returns:
;       SI, DI: Updated for next character
;   Corrupts registers:
;       AX, CX, DX
;--------------------------------------------------------------------
ALIGN DISPLAY_JUMP_ALIGN
CgaSnow_RepMovsb:
    call    LoadCgaStatusRegisterAddressToDXifCgaPresent
    jz      SHORT .RepMovsbWithoutWaitSinceUnknownPort

.MovsbNextByte:
    cli             ; Interrupt request would mess up timing
    WAIT_UNTIL_SAFE_CGA_WRITE
    es movsb
    sti
    loop    .MovsbNextByte
    ret
.RepMovsbWithoutWaitSinceUnknownPort:
    eSEG_STR rep, es, movsb
    ret


;--------------------------------------------------------------------
; LoadCgaStatusRegisterAddressToDXifCgaPresent
;   Parameters:
;       DS:     BDA segment (zero)
;   Returns:
;       DX:     CGA Status Register Address
;       ZF:     Set if CGA not present
;               Cleared if CGA present
;   Corrupts registers:
;       Nothing
;--------------------------------------------------------------------
ALIGN DISPLAY_JUMP_ALIGN
LoadCgaStatusRegisterAddressToDXifCgaPresent:
    test    BYTE [VIDEO_BDA.displayContext+DISPLAY_CONTEXT.bFlags], FLG_CONTEXT_CGA
    jz      SHORT .NoCgaDetected
    mov     dx, CGA_STATUS_REGISTER
ALIGN DISPLAY_JUMP_ALIGN, ret
.NoCgaDetected:
    ret


%endif ; ELIMINATE_CGA_SNOW