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diff --git a/docs/os12/mdfs.net/Docs/Comp/BBC/OS1-20/D4BF b/docs/os12/mdfs.net/Docs/Comp/BBC/OS1-20/D4BF new file mode 100644 index 0000000..2d10030 --- /dev/null +++ b/docs/os12/mdfs.net/Docs/Comp/BBC/OS1-20/D4BF @@ -0,0 +1 @@ +OS SERIES IV
GEOFF COX
*************************************************************************
* *
* LATERAL FILL ROUTINE *
* *
*************************************************************************
D4BF JSR &D4AA ;check current screen state
D4C2 AND &D1 ;if A and &D1 <> 0 a plotted point has been found
D4C4 BNE &D4B9 ;so D4B9
D4C6 LDX #&00 ;X=0
D4C8 JSR &D592 ;update pointers
D4CB BEQ &D4FA ;if 0 then D4FA
D4CD LDY &031A ;else Y=graphics scan line
D4D0 ASL &D1 ;
D4D2 BCS &D4D9 ;if carry set D4D9
D4D4 JSR &D574 ;else D574
D4D7 BCC &D4FA ;if carry clear D4FA
D4D9 JSR &D3FD ;else D3FD to pick up colour multiplier
D4DC LDA (&D6),Y ;get graphics cell line
D4DE EOR &035A ;EOR with background colour
D4E1 STA &DA ;and store
D4E3 BNE &D4F7 ;if not 0 D4F7
D4E5 SEC ;else set carry
D4E6 TXA ;A=X
D4E7 ADC &0361 ;add pixels/byte
D4EA BCC &D4F0 ;and if carry clear D4F0
D4EC INC &DB ;else increment &DB
D4EE BPL &D4F7 ;and if +ve D4F7
D4F0 TAX ;else X=A
D4F1 JSR &D104 ;display a pixel
D4F4 SEC ;set carry
D4F5 BCS &D4D9 ;goto D4D9
D4F7 JSR &D574 ;
D4FA LDY #&00 ;Y=0
D4FC JSR &D5AC ;
D4FF LDY #&20 ;
D501 LDX #&24 ;
D503 JSR &CDE6 ;exchange 300/3 +Y with 300/3+X
D506 JSR &D4AA ;check screen pixel
D509 LDX #&04 ;Y=5
D50B JSR &D592 ;
D50E TXA ;A=x
D50F BNE &D513 ;if A<>0 d513
D511 DEC &DB ;else &DB=&dB-1
D513 DEX ;X=X-1
D514 JSR &D54B ;
D517 BCC &D540 ;
D519 JSR &D3ED ;update pointers
D51C LDA (&D6),Y ;get byte from graphics line
D51E EOR &035A ;EOR with background colour
D521 STA &DA ;and store it
D523 LDA &DC ;
D525 BNE &D514 ;If A-0 back to D514
D527 LDA &DA ;else A=&DA
D529 BNE &D53D ;if A<>d53D
D52B SEC ;else set carry
D52C TXA ;A=x
D52D ADC &0361 ;Add number of pixels/byte
D530 BCC &D536 ;and if carry clear D536
D532 INC &DB ;else inc DB
D534 BPL &D53D ;and if +ve D53D
D536 TAX ;get back X
D537 JSR &D104 ;display a point
D53A SEC ;set carry
D53B BCS &D519 ;goto D519
D53D JSR &D54B ;
D540 LDY #&04 ;
D542 JSR &D5AC ;
D545 JSR &D0D9 ;
D548 JMP &D1B8 ;scale pointers
D54B LDA &D1 ;get byte mask
D54D PHA ;save it
D54E CLC ;clear carry
D54F BCC &D560 ;
D551 PLA ;get back A
D552 INX ;X=X+1
D553 BNE &D559 ;if not 0 D559
D555 INC &DB ;else inc &DB
D557 BPL &D56F ;if +ve D56F
D559 LSR &D1 ;
D55B BCS &D56F ;if Bit 7 D1 set D56F
D55D ORA &D1 ;else or withA
D55F PHA ;save result
D560 LDA &D1 ;A=&D1
D562 BIT &DA ;test bits 6 and 7 of &DA
D564 PHP ;save flags
D565 PLA ;get into A
D566 EOR &DC ;EOR and DC
D568 PHA ;save A
D569 PLP ;
D56A BEQ &D551 ;
D56C PLA ;A=A EOR &D1 (byte mask)
D56D EOR &D1 ;
D56F STA &D1 ;store it
D571 JMP &D0F0 ;and display a pixel
D574 LDA #&00 ;A=0
D576 CLC ;Clear carry
D577 BCC &D583 ;goto D583 if carry clear
D579 INX ;X=X+1
D57A BNE &D580 ;If <>0 D580
D57C INC &DB ;else inc &DB
D57E BPL &D56F ;and if +ve d56F
D580 ASL ;A=A*2
D581 BCS &D58E ;if C set D58E
D583 ORA &D1 ;else A=A OR (&D1)
D585 BIT &DA ;set V and M from &DA b6 b7
D587 BEQ &D579 ;
D589 EOR &D1 ;A=AEOR &D1
D58B LSR ;/2
D58C BCC &D56F ;if carry clear D56F
D58E ROR ;*2
D58F SEC ;set carry
D590 BCS &D56F ;to D56F
D592 LDA &0300,X ;Y/A=(&300/1 +X)-(&320/1)
D595 SEC ;
D596 SBC &0320 ;
D599 TAY ;
D59A LDA &0301,X ;
D59D SBC &0321 ;
D5A0 BMI &D5A5 ;if result -ve D5A5
D5A2 JSR &D49B ;or negate Y/A
D5A5 STA &DB ;store A
D5A7 TYA ;A=Y
D5A8 TAX ;X=A
D5A9 ORA &DB ;
D5AB RTS ;exit
;
D5AC STY &DA ;Y=&DA
D5AE TXA ;A=X
D5AF TAY ;Y=A
D5B0 LDA &DB ;A=&DB
D5B2 BMI &D5B6 ;if -ve D5B6
D5B4 LDA #&00 ;A=0
D5B6 LDX &DA ;X=&DA
D5B8 BNE &D5BD ;if <>0 D5BD
D5BA JSR &D49B ;negate
D5BD PHA ;
D5BE CLC ;
D5BF TYA ;
D5C0 ADC &0300,X ;Y/A+(&300/1 +X)=(&320/1)
D5C3 STA &0320 ;
D5C6 PLA ;
D5C7 ADC &0301,X ;
D5CA STA &0321 ;
D5CD RTS ;return
*************************************************************************
* *
* OSWORD 13 - READ LAST TWO GRAPHIC CURSOR POSITIONS *
* *
*************************************************************************
;
D5CE LDA #&03 ;A=3
D5D0 JSR &D5D5 ;
D5D3 LDA #&07 ;A=7
D5D5 PHA ;Save A
D5D6 JSR &CDE2 ;exchange last 2 graphics cursor coordinates with
;current coordinates
D5D9 JSR &D1B8 ;convert to external coordinates
D5DC LDX #&03 ;X=3
D5DE PLA ;save A
D5DF TAY ;Y=A
D5E0 LDA &0310,X ;get graphics coordinate
D5E3 STA (&F0),Y ;store it in OS buffer
D5E5 DEY ;decrement Y and X
D5E6 DEX ;
D5E7 BPL &D5E0 ;if +ve do it again
D5E9 RTS ;then Exit
;
*************************************************************************
* *
* PLOT Fill triangle routine *
* *
*************************************************************************
D5EA LDX #&20 ;X=&20
D5EC LDY #&3E ;Y=&3E
D5EE JSR &D47C ;copy 300/7+X to 300/7+Y
;this gets XY data parameters and current graphics
;cursor position
D5F1 JSR &D632 ;exchange 320/3 with 324/7 if 316/7=<322/3
D5F4 LDX #&14 ;X=&14
D5F6 LDY #&24 ;Y=&24
D5F8 JSR &D636 ;
D5FB JSR &D632 ;
D5FE LDX #&20 ;
D600 LDY #&2A ;
D602 JSR &D411 ;calculate 032A/B-(324/5-320/1)
D605 LDA &032B ;and store
D608 STA &0332 ;result
D60B LDX #&28 ;set pointers
D60D JSR &D459 ;
D610 LDY #&2E ;
D612 JSR &D0DE ;copy 320/3 32/31
D615 JSR &CDE2 ;exchange 314/7 with 324/7
D618 CLC ;
D619 JSR &D658 ;execute fill routine
D61C JSR &CDE2 ;
D61F LDX #&20 ;
D621 JSR &CDE4 ;
D624 SEC ;
D625 JSR &D658 ;
D628 LDX #&3E ;;X=&3E
D62A LDY #&20 ;;Y=&20
D62C JSR &D47C ;;copy 300/7+X to 300/7+Y
D62F JMP &D0D9 ;;this gets XY data parameters and current graphics
;cursor position
D632 LDX #&20 ;X=&20
D634 LDY #&14 ;Y=&14
D636 LDA &0302,X ;
D639 CMP &0302,Y ;
D63C LDA &0303,X ;
D63F SBC &0303,Y ;
D642 BMI &D657 ;if 302/3+Y>302/3+X return
D644 JMP &CDE6 ;else swap 302/3+X with 302/3+Y
;
*************************************************************************
* *
* OSBYTE 134 - READ CURSOR POSITION *
* *
*************************************************************************
D647 LDA &0318 ;read current text cursor (X)
D64A SEC ;set carry
D64B SBC &0308 ;subtract left hand column of current text window
D64E TAX ;X=A
D64F LDA &0319 ;get current text cursor (Y)
D652 SEC ;
D653 SBC &030B ;suptract top row of current window
D656 TAY ;Y=A
D657 RTS ;and exit
;PLOT routines continue
;many of the following routines are just manipulations
;only points of interest will be explained
D658 PHP ;store flags
D659 LDX #&20 ;X=&20
D65B LDY #&35 ;Y=&35
D65D JSR &D411 ;335/6=(324/5+X-320/1)
D660 LDA &0336 ;
D663 STA &033D ;
D666 LDX #&33 ;
D668 JSR &D459 ;set pointers
D66B LDY #&39 ;set 339/C=320/3
D66D JSR &D0DE ;
D670 SEC ;
D671 LDA &0322 ;
D674 SBC &0326 ;
D677 STA &031B ;
D67A LDA &0323 ;
D67D SBC &0327 ;
D680 STA &031C ;
D683 ORA &031B ;check VDU queque
D686 BEQ &D69F ;
D688 JSR &D6A2 ;display a line
D68B LDX #&33 ;
D68D JSR &D774 ;update pointers
D690 LDX #&28 ;
D692 JSR &D774 ;and again!
D695 INC &031B ;update VDU queque
D698 BNE &D688 ;and if not empty do it again
D69A INC &031C ;else increment next byte
D69D BNE &D688 ;and do it again
D69F PLP ;pull flags
D6A0 BCC &D657 ;if carry clear exit
D6A2 LDX #&39 ;
D6A4 LDY #&2E ;
D6A6 STX &DE ;
D6A8 LDA &0300,X ;is 300/1+x<300/1+Y
D6AB CMP &0300,Y ;
D6AE LDA &0301,X ;
D6B1 SBC &0301,Y ;
D6B4 BMI &D6BC ;if so D6BC
D6B6 TYA ;else A=Y
D6B7 LDY &DE ;Y=&DE
D6B9 TAX ;X=A
D6BA STX &DE ;&DE=X
D6BC STY &DF ;&DF=Y
D6BE LDA &0300,Y ;
D6C1 PHA ;
D6C2 LDA &0301,Y ;
D6C5 PHA ;
D6C6 LDX &DF ;
D6C8 JSR &D10F ;check for window violations
D6CB BEQ &D6DA ;
D6CD CMP #&02 ;
D6CF BNE &D70E ;
D6D1 LDX #&04 ;
D6D3 LDY &DF ;
D6D5 JSR &D482 ;
D6D8 LDX &DF ;
D6DA JSR &D864 ;set a screen address
D6DD LDX &DE ;X=&DE
D6DF JSR &D10F ;check for window violations
D6E2 LSR ;A=A/2
D6E3 BNE &D70E ;if A<>0 then exit
D6E5 BCC &D6E9 ;else if C clear D6E9
D6E7 LDX #&00 ;
D6E9 LDY &DF ;
D6EB SEC ;
D6EC LDA &0300,Y ;
D6EF SBC &0300,X ;
D6F2 STA &DC ;
D6F4 LDA &0301,Y ;
D6F7 SBC &0301,X ;
D6FA STA &DD ;
D6FC LDA #&00 ;
D6FE ASL ;
D6FF ORA &D1 ;
D701 LDY &DC ;
D703 BNE &D719 ;
D705 DEC &DD ;
D707 BPL &D719 ;
D709 STA &D1 ;
D70B JSR &D0F0 ;display a point
D70E LDX &DF ;restore X
D710 PLA ;and A
D711 STA &0301,X ;store it
D714 PLA ;get back A
D715 STA &0300,X ;and store it
D718 RTS ;exit
;
D719 DEC &DC ;
D71B TAX ;
D71C BPL &D6FE ;
D71E STA &D1 ;
D720 JSR &D0F0 ;display a point
D723 LDX &DC ;
D725 INX ;
D726 BNE &D72A ;
D728 INC &DD ;
D72A TXA ;
D72B PHA ;
D72C LSR &DD ;
D72E ROR ;
D72F LDY &0361 ;number of pixels/byte
D732 CPY #&03 ;if 3 mode = goto D73B
D734 BEQ &D73B ;
D736 BCC &D73E ;else if <3 mode 2 goto D73E
D738 LSR &DD ;else rotate bottom bit of &DD
D73A ROR ;into Accumulator
D73B LSR &DD ;rotate bottom bit of &DD
D73D LSR ;into Accumulator
D73E LDY &031A ;Y=line in current graphics cell containing current
;point
D741 TAX ;X=A
D742 BEQ &D753 ;
D744 TYA ;Y=Y-8
D745 SEC ;
D746 SBC #&08 ;
D748 TAY ;
D749 BCS &D74D ;
D74B DEC &D7 ;decrement byte of top line off current graphics cell
D74D JSR &D104 ;display a point
D750 DEX ;
D751 BNE &D744 ;
D753 PLA ;
D754 AND &0361 ;pixels/byte
D757 BEQ &D70E ;
D759 TAX ;
D75A LDA #&00 ;A=0
D75C ASL ;
D75D ORA &0363 ;or with right colour mask
D760 DEX ;
D761 BNE &D75C ;
D763 STA &D1 ;store as byte mask
D765 TYA ;Y=Y-8
D766 SEC ;
D767 SBC #&08 ;
D769 TAY ;
D76A BCS &D76E ;if carry clear
D76C DEC &D7 ;decrement byte of top line off current graphics cell
D76E JSR &D0F3 ;display a point
D771 JMP &D70E ;and exit via D70E
D774 INC &0308,X ;
D777 BNE &D77C ;
D779 INC &0309,X ;
D77C SEC ;
D77D LDA &0300,X ;
D780 SBC &0302,X ;
D783 STA &0300,X ;
D786 LDA &0301,X ;
D789 SBC &0303,X ;
D78C STA &0301,X ;
D78F BPL &D7C1 ;
D791 LDA &030A,X ;
D794 BMI &D7A1 ;
D796 INC &0306,X ;
D799 BNE &D7AC ;
D79B INC &0307,X ;
D79E JMP &D7AC ;
D7A1 LDA &0306,X ;
D7A4 BNE &D7A9 ;
D7A6 DEC &0307,X ;
D7A9 DEC &0306,X ;
D7AC CLC ;
D7AD LDA &0300,X ;
D7B0 ADC &0304,X ;
D7B3 STA &0300,X ;
D7B6 LDA &0301,X ;
D7B9 ADC &0305,X ;
D7BC STA &0301,X ;
D7BF BMI &D791 ;
D7C1 RTS ;
;
;
*************************************************************************
* *
* OSBYTE 135 - READ CHARACTER AT TEXT CURSOR POSITION *
* *
*************************************************************************
D7C2 LDY &0360 ;get number of logical colours
D7C5 BNE &D7DC ;if Y<>0 mode <>7 so D7DC
D7C7 LDA (&D8),Y ;get address of top scan line of current text chr
D7C9 LDY #&02 ;Y=2
D7CB CMP &C4B7,Y ;compare with conversion table
D7CE BNE &D7D4 ;if not equal D7d4
D7D0 LDA &C4B6,Y ;else get next lower byte from table
D7D3 DEY ;Y=Y-1
D7D4 DEY ;Y=Y-1
D7D5 BPL &D7CB ;and if +ve do it again
D7D7 LDY &0355 ;Y=current screen mode
D7DA TAX ;return with character in X
D7DB RTS ;
;
D7DC JSR &D808 ;set up copy of the pattern bytes at text cursor
D7DF LDX #&20 ;X=&20
D7E1 TXA ;A=&20
D7E2 PHA ;Save it
D7E3 JSR &D03E ;get pattern address for code in A
D7E6 PLA ;get back A
D7E7 TAX ;and X
D7E8 LDY #&07 ;Y=7
D7EA LDA &0328,Y ;get byte in pattern copy
D7ED CMP (&DE),Y ;check against pattern source
D7EF BNE &D7F9 ;if not the same D7F9
D7F1 DEY ;else Y=Y-1
D7F2 BPL &D7EA ;and if +ve D7EA
D7F4 TXA ;A=X
D7F5 CPX #&7F ;is X=&7F (delete)
D7F7 BNE &D7D7 ;if not D7D7
D7F9 INX ;else X=X+1
D7FA LDA &DE ;get byte lo address
D7FC CLC ;clear carry
D7FD ADC #&08 ;add 8
D7FF STA &DE ;store it
D801 BNE &D7E8 ;and go back to check next character if <>0
D803 TXA ;A=X
D804 BNE &D7E1 ;if <>0 D7E1
D806 BEQ &D7D7 ;else D7D7
***************** set up pattern copy ***********************************
D808 LDY #&07 ;Y=7
D80A STY &DA ;&DA=Y
D80C LDA #&01 ;A=1
D80E STA &DB ;&DB=A
D810 LDA &0362 ;A=left colour mask
D813 STA &DC ;store an &DC
D815 LDA (&D8),Y ;get a byte from current text character
D817 EOR &0358 ;EOR with text background colour
D81A CLC ;clear carry
D81B BIT &DC ;and check bits of colour mask
D81D BEQ &D820 ;if result =0 then D820
D81F SEC ;else set carry
D820 ROL &DB ;&DB=&DB+Carry
D822 BCS &D82E ;if carry now set (bit 7 DB originally set) D82E
D824 LSR &DC ;else &DC=&DC/2
D826 BCC &D81B ;if carry clear D81B
D828 TYA ;A=Y
D829 ADC #&07 ;ADD ( (7+carry)
D82B TAY ;Y=A
D82C BCC &D810 ;
D82E LDY &DA ;read modified values into Y and A
D830 LDA &DB ;
D832 STA &0328,Y ;store copy
D835 DEY ;and do it again
D836 BPL &D80A ;until 8 bytes copied
D838 RTS ;exit
;
********* pixel reading *************************************************
D839 PHA ;store A
D83A TAX ;X=A
D83B JSR &D149 ;set up positional data
D83E PLA ;get back A
D83F TAX ;X=A
D840 JSR &D85F ;set a screen address after checking for window
;violations
D843 BNE &D85A ;if A<>0 D85A to exit with A=&FF
D845 LDA (&D6),Y ;else get top line of current graphics cell
D847 ASL ;A=A*2 C=bit 7
D848 ROL &DA ;&DA=&DA+2 +C C=bit 7 &DA
D84A ASL &D1 ;byte mask=bM*2 +carry from &DA
D84C PHP ;save flags
D84D BCS &D851 ;if carry set D851
D84F LSR &DA ;else restore &DA with bit '=0
D851 PLP ;pull flags
D852 BNE &D847 ;if Z set D847
D854 LDA &DA ;else A=&DA AND number of colours in current mode -1
D856 AND &0360 ;
D859 RTS ;then exit
;
D85A LDA #&FF ;A=&FF
D85C RTS ;exit
;
********** check for window violations and set up screen address **********
D85D LDX #&20 ;X=&20
D85F JSR &D10F ;
D862 BNE &D85C ;if A<>0 there is a window violation so D85C
D864 LDA &0302,X ;else set up graphics scan line variable
D867 EOR #&FF ;
D869 TAY ;
D86A AND #&07 ;
D86C STA &031A ;in 31A
D86F TYA ;A=Y
D870 LSR ;A=A/2
D871 LSR ;A=A/2
D872 LSR ;A=A/2
D873 ASL ;A=A*2 this gives integer value bit 0 =0
D874 TAY ;Y=A
D875 LDA (&E0),Y ;get high byte of offset from screen RAM start
D877 STA &DA ;store it
D879 INY ;Y=Y+1
D87A LDA (&E0),Y ;get lo byte
D87C LDY &0356 ;get screen map type
D87F BEQ &D884 ;if 0 (modes 0,1,2) goto D884
D881 LSR &DA ;else &DA=&DA/2
D883 ROR ;and A=A/2 +C if set
;so 2 byte offset =offset/2
D884 ADC &0350 ;add screen top left hand corner lo
D887 STA &D6 ;store it
D889 LDA &DA ;get high byte
D88B ADC &0351 ;add top left hi
D88E STA &D7 ;store it
D890 LDA &0301,X ;
D893 STA &DA ;
D895 LDA &0300,X ;
D898 PHA ;
D899 AND &0361 ;and then Add pixels per byte-1
D89C ADC &0361 ;
D89F TAY ;Y=A
D8A0 LDA &C406,Y ;A=&80 /2^Y using look up table
D8A3 STA &D1 ;store it
D8A5 PLA ;get back A
D8A6 LDY &0361 ;Y=&number of pixels/byte
D8A9 CPY #&03 ;is Y=3 (modes 1,6)
D8AB BEQ &D8B2 ;goto D8B2
D8AD BCS &D8B5 ;if mode =1 or 4 D8B5
D8AF ASL ;A/&DA =A/&DA *2
D8B0 ROL &DA ;
D8B2 ASL ;
D8B3 ROL &DA ;
D8B5 AND #&F8 ;clear bits 0-2
D8B7 CLC ;clear carry
D8B8 ADC &D6 ;add A/&DA to &D6/7
D8BA STA &D6 ;
D8BC LDA &DA ;
D8BE ADC &D7 ;
D8C0 BPL &D8C6 ;if result +ve D8C6
D8C2 SEC ;else set carry
D8C3 SBC &0354 ;and subtract screen memory size making it wrap round
D8C6 STA &D7 ;store it in &D7
D8C8 LDY &031A ;get line in graphics cell containing current graphics
D8CB LDA #&00 ;point A=0
D8CD RTS ;And exit
;
D8CE PHA ;Push A
D8CF LDA #&A0 ;A=&A0
D8D1 LDX &026A ;X=number of items in VDU queque
D8D4 BNE &D916 ;if not 0 D916
D8D6 BIT &D0 ;else check VDU status byte
D8D8 BNE &D916 ;if either VDU is disabled or plot to graphics
;cursor enabled then D916
D8DA BVS &D8F5 ;if cursor editing enabled D8F5
D8DC LDA &035F ;else get 6845 register start setting
D8DF AND #&9F ;clear bits 5 and 6
D8E1 ORA #&40 ;set bit 6 to modify last cursor size setting
D8E3 JSR &C954 ;change write cursor format
D8E6 LDX #&18 ;X=&18
D8E8 LDY #&64 ;Y=&64
D8EA JSR &D482 ;set text input cursor from text output cursor
D8ED JSR &CD7A ;modify character at cursor poistion
D8F0 LDA #&02 ;A=2
D8F2 JSR &C59D ;bit 1 of VDU status is set to bar scrolling
D8F5 LDA #&BF ;A=&BF
D8F7 JSR &C5A8 ;bit 6 of VDU status =0
D8FA PLA ;Pull A
D8FB AND #&7F ;clear hi bit (7)
D8FD JSR &C4C0 ;entire VDU routine !!
D900 LDA #&40 ;A=&40
D902 JMP &C59D ;exit
D905 LDA #&20 ;A=&20
D907 BIT &D0 ;if bit 6 cursor editing is set
D909 BVC &D8CB ;
D90B BNE &D8CB ;or bit 5 is set exit &D8CB
D90D JSR &D7C2 ;read a character from the screen
D910 BEQ &D917 ;if A=0 on return exit via D917
D912 PHA ;else store A
D913 JSR &C664 ;perform cursor right
D916 PLA ;restore A
D917 RTS ;and exit
;
D918 LDA #&BD ;zero bits 2 and 6 of VDU status
D91A JSR &C5A8 ;
D91D JSR &C951 ;set normal cursor
D920 LDA #&0D ;A=&0D
D922 RTS ;and return
;this is response of CR as end of edit line
*************************************************************************
* *
* OSBYTE 132 - READ BOTTOM OF DISPLAY RAM *
* *
*************************************************************************
;
D923 LDX &0355 ; Get current screen mode
*************************************************************************
* *
* OSBYTE 133 - READ LOWEST ADDRESS FOR GIVEN MODE *
* *
*************************************************************************
D926 TXA ; A=X
D927 AND #&07 ; Ensure mode 0-7
D929 TAY ; Pass to Y into index into screen size table
D92A LDX &C440,Y ; X=screen size type, 0-4
D92D LDA &C45E,X ; A=high byte of start address for screen type
D930 LDX #&00 ; Returned address is &xx00
D932 BIT &028E ; Check available RAM
D935 BMI &D93E ; If bit 7 set then 32K RAM, so return address
D937 AND #&3F ; 16K RAM, so drop address to bottom 16K
D939 CPY #&04 ; Check screen mode
D93B BCS &D93E ; If mode 4-7, return the address
D93D TXA ; If mode 0-3, return &0000 as not enough memory
; exit
D93E TAY ; Pass high byte of address to Y
D93F RTS ; and return address in YX
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