BBC Operation System OS 1.20		VDU Main Routines

**************************************************************************
**************************************************************************
**                                                                      **
**      OSWRCH  MAIN ROUTINE  entry from E0C5                           **
**                                                                      **
**      output a byte via the VDU stream                                **
**                                                                      **
**************************************************************************
**************************************************************************
;This routine takes up over 40% of the operating system ROM
;entry points are variable, as are the results achieved.
;tracing any particular path is relatively easy but generalising for
;commenting is not.  For clarity comments will not be as detailed as
;for later parts of the Operating system.

C4C0	LDX &026A   ;get number of items in VDU queue
C4C3	BNE &C512   ;if parameters needed then C512
C4C5	BIT &D0     ;else check status byte
C4C7	BVC &C4D8   ;if cursor editing enabled two cursors exist
C4C9	JSR &C568   ;swap values
C4CC	JSR &CD6A   ;then set up write cursor
C4CF	BMI &C4D8   ;if display disabled C4D8
C4D1	CMP #&0D    ;else if character in A=RETURN teminate edit
C4D3	BNE &C4D8   ;else C4D8

C4D5	JSR &D918   ;terminate edit

C4D8	CMP #&7F    ;is character DELETE ?
C4DA	BEQ &C4ED   ;if so C4ED

C4DC	CMP #&20    ;is it less than space? (i.e. VDU control code)
C4DE	BCC &C4EF   ;if so C4EF
C4E0	BIT &D0     ;else check VDU byte ahain
C4E2	BMI &C4EA   ;if screen disabled C4EA
C4E4	JSR &CFB7   ;else display a character
C4E7	JSR &C664   ;and cursor right
C4EA	JMP &C55E   ;

********* read link addresses and number of parameters *****************

C4ED	LDA #&20    ;to replace delete character

********* read link addresses and number of parameters *****************

C4EF	TAY         ;Y=A
C4F0	LDA &C333,Y ;get lo byte of link address
C4F3	STA &035D   ;store it in jump vector
C4F6	LDA &C354,Y ;get hi byte
C4F9	BMI &C545   ;if negative (as it will be if a direct address)
    	            ;there are no parameters needed
    	            ;so C545
C4FB	TAX         ;else X=A
C4FC	ORA #&F0    ;set up negated parameter count
C4FE	STA &026A   ;store it as number of items in VDU queue
C501	TXA         ;get back A
C502	LSR         ;A=A/16
C503	LSR         ;
C504	LSR         ;
C505	LSR         ;
C506	CLC         ;clear carry
C507	ADC #&C3    ;add &C3 to get hi byte of link address
C509	STA &035E   ;
C50C	BIT &D0     ;check if cursor editing enabled
C50E	BVS &C52F   ;if so re-exchange pointers
C510	CLC         ;clear carry
C511	RTS         ;and exit

;return with carry clear indicates that printer action not required.
;
********** parameters are outstanding ***********************************
X=&26A = 2 complement of number of parameters X=&FF for 1, FE for 2 etc.

C512	STA &0224,X ;store parameter in queue
C515	INX         ;increment X
C516	STX &026A   ;store it as VDU queue
C519	BNE &C532   ;if not 0 C532 as more parameters are needed
C51B	BIT &D0     ;get VDU status byte
C51D	BMI &C534   ;if screen disabled C534
C51F	BVS &C526   ;else if cursor editing C526
C521	JSR &CCF5   ;execute required function
C524	CLC         ;clear carry
C525	RTS         ;and exit
;
C526	JSR &C568   ;swap values of cursors
C529	JSR &CD6A   ;set up write cursor
C52C	JSR &CCF5   ;execute required function
C52F	JSR &C565   ;re-exchange pointers

C532	CLC         ;carry clear
C533	RTS         ;exit

*************************************************************************
*                                                                       *
*       VDU 1 - SEND NEXT CHARACTER TO PRINTER                          *
*                                                                       *
*       1 parameter required                                            *
*                                                                       *
*************************************************************************
;
C534	LDY &035E   ;if upper byte of link address not &C5
C537	CPY #&C5    ;printer is not interested
C539	BNE &C532   ;so C532
C53B	TAX         ;else X=A
C53C	LDA &D0     ;A=VDU status byte
C53E	LSR         ;get bit 0 into carry
C53F	BCC &C511   ;if printer not enabled exit
C541	TXA         ;restore A
C542	JMP &E11E   ;else send byte in A (next byte) to printer

*********** if explicit link address found, no parameters ***************

C545	STA &035E   ;upper byte of link address
C548	TYA         ;restore A
C549	CMP #&08    ;is it 7 or less?
C54B	BCC &C553   ;if so C553
C54D	EOR #&FF    ;invert it
C54F	CMP #&F2    ;c is set if A >&0D
C551	EOR #&FF    ;re invert

C553	BIT &D0     ;VDU status byte
C555	BMI &C580   ;if display disabled C580
C557	PHP         ;push processor flags
C558	JSR &CCF5   ;execute required function
C55B	PLP         ;get back flags
C55C	BCC &C561   ;if carry clear (from C54B/F)

**************** main exit routine **************************************

C55E	LDA &D0     ;VDU status byte
C560	LSR         ;Carry is set if printer is enabled
C561	BIT &D0     ;VDU status byte
C563	BVC &C511   ;if no cursor editing  C511 to exit

***************** cursor editing routines *******************************

C565	JSR &CD7A   ;restore normal write cursor

C568	PHP         ;save flags and
C569	PHA         ;A
C56A	LDX #&18    ;X=&18
C56C	LDY #&64    ;Y=&64
C56E	JSR &CDDE   ;exchange &300/1+X with &300/1+Y
C571	JSR &CF06   ;set up display address
C574	JSR &CA02   ;set cursor position
C577	LDA &D0     ;VDU status byte
C579	EOR #&02    ;invert bit 1 to allow or bar scrolling
C57B	STA &D0     ;VDU status byte
C57D	PLA         ;restore flags and A
C57E	PLP         ;
C57F	RTS         ;and exit
;
C580	EOR #&06    ;if A<>6
C582	BNE &C58C   ;return via C58C
C584	LDA #&7F    ;A=&7F
C586	BCC &C5A8   ;and goto C5A8 ALWAYS!!

******************* check text cursor in use ***************************

C588	LDA &D0     ;VDU status byte
C58A	AND #&20    ;set A from bit 5 of status byte
C58C	RTS         ;and exit

A=0 if text cursor, &20 if graphics

*************************************************************************
*                                                                       *
*       VDU 14 - SET PAGED MODE                                         *
*                                                                       *
*************************************************************************
;
C58D	LDY #&00    ;Y=0
C58F	STY &0269   ;paged mode counter
C592	LDA #&04    ;A=04
C594	BNE &C59D   ;jump to C59D

*************************************************************************
*                                                                       *
*       VDU 2  - PRINTER ON (START PRINT JOB)                           *
*                                                                       *
*************************************************************************

C596	JSR &E1A2   ;select printer buffer and output character
C599	LDA #&94    ;A=&94
    	            ;when inverted at C59B this becomes =&01

*************************************************************************
*                                                                       *
*       VDU 21 - DISABLE DISPLAY                                        *
*                                                                       *
*************************************************************************

C59B	EOR #&95    ;if A=&15 A now =&80: if A=&94 A now =1

C59D	ORA &D0     ;VDU status byte set bit 0 or bit 7
C59F	BNE &C5AA   ;branch forward to store



*************************************************************************
*                                                                       *
*       VDU 3  - PRINTER OFF (END PRINT JOB)                            *
*                                                                       *
*************************************************************************

C5A1	JSR &E1A2   ;select printer buffer and output character
C5A4	LDA #&0A    ;A=10 to clear status bits below...

*************************************************************************
*                                                                       *
*       VDU 15 - PAGED MODE OFF                                         *
*                                                                       *
*************************************************************************
; A=&F or &A

C5A6	EOR #&F4    ;convert to &FB or &FE
C5A8	AND &D0     ;VDU status byte clear bit 0 or bit 2 of status
C5AA	STA &D0     ;VDU status byte
C5AC	RTS         ;exit

*************************************************************************
*                                                                       *
*       VDU 4  - OUTPUT AT TEXT CURSOR                                  *
*                                                                       *
*************************************************************************
;
C5AD	LDA &0361   ;pixels per byte
C5B0	BEQ &C5AC   ;if no graphics in current mode C5AC
C5B2	JSR &C951   ;set CRT controller for text cursor
C5B5	LDA #&DF    ;this to clear bit 5 of status byte
C5B7	BNE &C5A8   ;via C5A8 exit

*************************************************************************
*                                                                       *
*       VDU 5  - OUTPUT AT GRAPHICS CURSOR                              *
*                                                                       *
*************************************************************************

C5B9	LDA &0361   ;pixels per byte
C5BC	BEQ &C5AC   ;if none this is text mode so exit
C5BE	LDA #&20    ;set up graphics cursor
C5C0	JSR &C954   ;via C954
C5C3	BNE &C59D   ;set bit 5 via exit C59D

*************************************************************************
*                                                                       *
*       VDU 8  - CURSOR LEFT                                            *
*                                                                       *
*************************************************************************

C5C5	JSR &C588   ;A=0 if text cursor A=&20 if graphics cursor
C5C8	BNE &C61F   ;move cursor left 8 pixels if graphics
C5CA	DEC &0318   ;else decrement text column
C5CD	LDX &0318   ;store new text column
C5D0	CPX &0308   ;if it is less than text window left
C5D3	BMI &C5EE   ;do wraparound  cursor to rt of screen 1 line up
C5D5	LDA &034A   ;text cursor 6845 address
C5D8	SEC         ;subtract
C5D9	SBC &034F   ;bytes per character
C5DC	TAX         ;put in X
C5DD	LDA &034B   ;get text cursor 6845 address
C5E0	SBC #&00    ;subtract 0
C5E2	CMP &034E   ;compare with hi byte of screen RAM address
C5E5	BCS &C5EA   ;if = or greater
C5E7	ADC &0354   ;add screen RAM size hi byte to wrap around
C5EA	TAY         ;Y=A
C5EB	JMP &C9F6   ;Y hi and X lo byte of cursor position

***************** execute wraparound left-up*****************************

C5EE	LDA &030A   ;text window right
C5F1	STA &0318   ;text column

*************** cursor up ***********************************************

C5F4	DEC &0269   ;paged mode counter
C5F7	BPL &C5FC   ;if still greater than 0 skip next instruction
C5F9	INC &0269   ;paged mode counter to restore X=0
C5FC	LDX &0319   ;current text line
C5FF	CPX &030B   ;top of text window
C602	BEQ &C60A   ;if its at top of window C60A
C604	DEC &0319   ;else decrement current text line
C607	JMP &C6AF   ;and carry on moving cursor

******** cursor at top of window ****************************************

C60A	CLC         ;clear carry
C60B	JSR &CD3F   ;check for window violatations
C60E	LDA #&08    ;A=8 to check for software scrolling
C610	BIT &D0     ;compare against VDU status byte
C612	BNE &C619   ;if not enabled C619
C614	JSR &C994   ;set screen start register and adjust RAM
C617	BNE &C61C   ;jump C61C

C619	JSR &CDA4   ;soft scroll 1 line
C61C	JMP &C6AC   ;and exit

**********cursor left and down with graphics cursor in use **************

C61F	LDX #&00    ;X=0 to select horizontal parameters

********** cursor down with graphics in use *****************************
;X=2 for vertical or 0 for horizontal

C621	STX &DB     ;store X
C623	JSR &D10D   ;check for window violations
C626	LDX &DB     ;restore X
C628	SEC         ;set carry
C629	LDA &0324,X ;current graphics cursor X>1=vertical
C62C	SBC #&08    ;subtract 8 to move back 1 character
C62E	STA &0324,X ;store in current graphics cursor X>1=verticaal
C631	BCS &C636   ;if carry set skip next
C633	DEC &0325,X ;current graphics cursor hi -1
C636	LDA &DA     ;&DA=0 if no violation else 1 if vert violation
    	            ;2 if horizontal violation
C638	BNE &C658   ;if violation C658
C63A	JSR &D10D   ;check for window violations
C63D	BEQ &C658   ;if none C658

C63F	LDX &DB     ;else get back X
C641	LDA &0304,X ;graphics window rt X=0 top X=2
C644	CPX #&01    ;is X=0
C646	BCS &C64A   ;if not C64A
C648	SBC #&06    ;else subtract 7

C64A	STA &0324,X ;current graphics cursor X>1=vertical
C64D	LDA &0305,X ;graphics window hi rt X=0 top X=2
C650	SBC #&00    ;subtract carry
C652	STA &0325,X ;current graphics cursor X<2=horizontal else vertical
C655	TXA         ;A=X
C656	BEQ &C660   ;cursor up
C658	JMP &D1B8   ;set up external coordinates for graphics

*************************************************************************
*                                                                       *
*       VDU 11 - CURSOR UP                                              *
*                                                                       *
*************************************************************************

C65B	JSR &C588   ;A=0 if text cursor A=&20 if graphics cursor
C65E	BEQ &C5F4   ;if text cursor then C5F4
C660	LDX #&02    ;else X=2
C662	BNE &C6B6   ;goto C6B6

*************************************************************************
*                                                                       *
*       VDU 9  - CURSOR RIGHT                                           *
*                                                                       *
*************************************************************************

C664	LDA &D0     ;VDU status byte
C666	AND #&20    ;check bit 5
C668	BNE &C6B4   ;if set then graphics cursor in use so C6B4
C66A	LDX &0318   ;text column
C66D	CPX &030A   ;text window right
C670	BCS &C684   ;if X exceeds window right then C684
C672	INC &0318   ;text column
C675	LDA &034A   ;text cursor 6845 address
C678	ADC &034F   ;add bytes per character
C67B	TAX         ;X=A
C67C	LDA &034B   ;text cursor 6845 address
C67F	ADC #&00    ;add carry if set
C681	JMP &C9F6   ;use X and Y to set new cursor address

********: text cursor down and right *************************************

C684	LDA &0308   ;text window left
C687	STA &0318   ;text column

********: text cursor down *************************************

C68A	CLC         ;clear carry
C68B	JSR &CAE3   ;check bottom margin, X=line count
C68E	LDX &0319   ;current text line
C691	CPX &0309   ;bottom margin
C694	BCS &C69B   ;if X=>current bottom margin C69B
C696	INC &0319   ;else increment current text line
C699	BCC &C6AF   ;
C69B	JSR &CD3F   ;check for window violations
C69E	LDA #&08    ;check bit 3
C6A0	BIT &D0     ;VDU status byte
C6A2	BNE &C6A9   ;if software scrolling enabled C6A9
C6A4	JSR &C9A4   ;perform hardware scroll
C6A7	BNE &C6AC   ;
C6A9	JSR &CDFF   ;execute upward scroll
C6AC	JSR &CEAC   ;clear a line

C6AF	JSR &CF06   ;set up display address
C6B2	BCC &C732   ;






*********** graphic cursor right ****************************************

C6B4	LDX #&00    ;

************** graphic cursor up  (X=2) **********************************

C6B6	STX &DB     ;store X
C6B8	JSR &D10D   ;check for window violations
C6BB	LDX &DB     ;get back X
C6BD	CLC         ;clear carry
C6BE	LDA &0324,X ;current graphics cursor X>1=vertical
C6C1	ADC #&08    ;Add 8 pixels
C6C3	STA &0324,X ;current graphics cursor X>1=vertical
C6C6	BCC &C6CB   ;
C6C8	INC &0325,X ;current graphics cursor X<2=horizontal else vertical
C6CB	LDA &DA     ;A=0 no window violations 1 or 2 indicates violation
C6CD	BNE &C658   ;if outside window C658
C6CF	JSR &D10D   ;check for window violations
C6D2	BEQ &C658   ;if no violations C658

C6D4	LDX &DB     ;get back X
C6D6	LDA &0300,X ;graphics window X<2 =left else bottom
C6D9	CPX #&01    ;If X=0
C6DB	BCC &C6DF   ;C6DF
C6DD	ADC #&06    ;else add 7
C6DF	STA &0324,X ;current graphics cursor X>1=vertical
C6E2	LDA &0301,X ;graphics window hi X<2 =left else bottom
C6E5	ADC #&00    ;add anny carry
C6E7	STA &0325,X ;current graphics cursor X<2=horizontal else vertical
C6EA	TXA         ;A=X
C6EB	BEQ &C6F5   ;if X=0 C6F5 cursor down
C6ED	JMP &D1B8   ;set up external coordinates for graphics

*************************************************************************
*                                                                       *
*       VDU 10  - CURSOR DOWN                                           *
*                                                                       *
*************************************************************************

C6F0	JSR &C588   ;A=0 if text cursor A=&20 if graphics cursor
C6F3	BEQ &C68A   ;if text cursor back to C68A
C6F5	LDX #&02    ;else X=2 to indicate vertical movement
C6F7	JMP &C621   ;move graphics cursor down

*************************************************************************
*                                                                       *
*       VDU 28 - DEFINE TEXT WINDOW                                     *
*                                                                       *
*       4 parameters                                                    *
*                                                                       *
*************************************************************************
;parameters are set up thus
;0320  P1 left margin
;0321  P2 bottom margin
;0322  P3 right margin
;0323  P4 top margin
;Note that last parameter is always in 0323

C6FA	LDX &0355   ;screen mode
C6FD	LDA &0321   ;get bottom margin
C700	CMP &0323   ;compare with top margin
C703	BCC &C758   ;if bottom margin exceeds top return
C705	CMP &C3E7,X ;text window bottom margin maximum
C708	BEQ &C70C   ;if equal then its OK
C70A	BCS &C758   ;else exit



C70C	LDA &0322   ;get right margin
C70F	TAY         ;put it in Y
C710	CMP C3EF,X  ;text window right hand margin maximum
C713	BEQ &C717   ;if equal then OK
C715	BCS &C758   ;if greater than maximum exit

C717	SEC         ;set carry to subtract
C718	SBC &0320   ;left margin
C71B	BMI &C758   ;if left greater than right exit
C71D	TAY         ;else A=Y (window width)
C71E	JSR &CA88   ;calculate number of bytes in a line
C721	LDA #&08    ;A=8 to set bit  of &D0
C723	JSR &C59D   ;indicating that text window is defined
C726	LDX #&20    ;point to parameters
C728	LDY #&08    ;point to text window margins
C72A	JSR &D48A   ;(&300/3+Y)=(&300/3+X)
C72D	JSR &CEE8   ;set up screen address
C730	BCS &C779   ;home cursor within window
C732	JMP &CA02   ;set cursor position