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Thread: CGA card issue (freeze)

  1. #11
    Join Date
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    Only in 80x25 text mode. From the BIOS source:

    Code:
    ;---- HANDLE COMMON SCROLL SET UP HERE
    
    SCROLL_POSITION PROC	NEAR
    	CMP	CRT_MODE,2		; TEST FOR SPECIAL CASE HERE
    	JB	N9			; HAVE TO HANDLE 80X25 SEPARATELY
    	CMP	CRT_MODE,3
    	JA	N9
    
    ;----- 80X25 COLOR CARD SCROLL
    
    	PUSH	DX
    	MOV	DX,3DAH			; GUARANTEED TO BE COLOR CARD HERE
    	PUSH	AX
    N8:					; WAIT_DISP_ENABLE
    	IN	AL,DX			; GET PORT
    	TEST	AL,8			; WAIT FOR VERTICAL RETRACE
    	JZ	N8			; WAIT_DISP_ENABLE
    	MOV	AL,25H
    	MOV	DL,0D8H			; DX=3D8
    	OUT	DX,AL			; TURN OFF VIDEO
    	POP	AX			; DURING VERTICAL RETRACE
    	POP	DX
    N9:
    	CALL	POSITION		; CONVERT TO REGEN POINTER
    	ADD	AX,CRT_START		; OFFSET OF ACTIVE PAGE
    	MOV	DI,AX			; TO ADDRESS FOR SCROLL
    	MOV	SI,AX			; FROM ADDRESS FOR SCROLL
    	SUB	DX,CX			; DX = # ROWS, # COLS IN BLOCK
    	INC	DH
    	INC	DL			; INCREMENT FOR 0 ORIGIN
    	XOR	CH,CH			; SET HIGH BYTE OF COUNT TO ZERO
    	MOV	BP,CRT_COLS		; GET NUMBER OF COLUMNS IN DISPLAY
    	ADD	BP,BP			; TIMES 2 FOR ATTRIBUTE BYTE
    	MOV	AL,BL			; GET LINE COUNT
    	MUL	BYTE PTR CRT_COLS	; DETERMINE OFFSET TO FROM ADDRESS
    	ADD	AX,AX			; *2 FOR ATTRIBUTE BYTE
    	PUSH	ES			; ESTABLISH ADDRESSING TO REGEN BUFFER
    	POP	DS			; FOR BOTH POINTERS
    	CMP	BL,0			; 0 SCROLL MEANS BLANK FIELD
    	RET				; RETURN WITH FLAGS SET
    SCROLL_POSITION ENDP
    You can see how, if the retrace status is missing, an 80x25 scroll would freeze solid.

  2. #12
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    So in addition to the contents of the CONFIG.SYS and AUTOEXEC.BAT, also type "MODE CO40" and then see if the scrolling issue persists.
    Offering a bounty for:
    - The software "Overhead Express" (doesn't have to be original, can be a copy)
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  3. #13
    Join Date
    Jul 2019
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    France, Pont--Mousson
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    Quote Originally Posted by VileR View Post
    I don't recall, but does the 5160 BIOS check for retrace even in 40-column mode? After all there should be no snow problem there.

    If it doesn't, that could help test for this specific problem at least.
    Yes ! problem solved by 40-column mode ! Thank you for this tips, now I can use my original monitor with CGA card !

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