From b1969ac16a656d9cd18e755b9e0e6b07085befe4 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Thu, 19 Feb 2015 22:19:21 +0100 Subject: [PATCH 01/14] kbhit() function and scrolling. Patch provided by Jeff Tranter. --- libsrc/osic1p/cputc.s | 12 ++++++++++-- libsrc/osic1p/kbhit.s | 28 ++++++++++++++++++++++++++++ libsrc/osic1p/osic1p.inc | 1 + 3 files changed, 39 insertions(+), 2 deletions(-) create mode 100644 libsrc/osic1p/kbhit.s diff --git a/libsrc/osic1p/cputc.s b/libsrc/osic1p/cputc.s index f6f285301..47969df15 100644 --- a/libsrc/osic1p/cputc.s +++ b/libsrc/osic1p/cputc.s @@ -48,8 +48,16 @@ newline: lda CURS_Y cmp #SCR_HEIGHT ; screen height bne plot - lda #0 ; wrap around to line 0 - sta CURS_Y + dec CURS_Y ; bottom of screen reached, scroll + ldx #0 +scroll: lda SCRNBASE+$00A5,x + sta SCRNBASE+$0085,x + lda SCRNBASE+$01A5,x + sta SCRNBASE+$0185,x + lda SCRNBASE+$02A5,x + sta SCRNBASE+$0285,x + inx + bne scroll plot: ldy CURS_Y lda ScrLo,y diff --git a/libsrc/osic1p/kbhit.s b/libsrc/osic1p/kbhit.s new file mode 100644 index 000000000..c064fec47 --- /dev/null +++ b/libsrc/osic1p/kbhit.s @@ -0,0 +1,28 @@ +; +; unsigned char kbhit (void); +; + + .export _kbhit + .include "osic1p.inc" + +_kbhit: + lda #%11111110 ; Select first keyboard row +scan: + sta KBD ; Select keyboard row + tax ; Save A + lda KBD ; Read keyboard columns + ora #$01 ; Mask out lsb (Shift Lock), since we ignore it + cmp #$FF ; No keys pressed? + bne keypressed + txa ; Restore A + sec ; Want to shift in ones + rol a ; Rotate row select to next bit position + cmp #$FF ; Done? + bne scan ; If not, continue + ldx #$00 ; High byte of return is always zero + lda #$00 ; Return false + rts +keypressed: + ldx #$00 ; High byte of return is always zero + lda #$01 ; Return true + rts diff --git a/libsrc/osic1p/osic1p.inc b/libsrc/osic1p/osic1p.inc index 9a0c346b6..e95db637e 100644 --- a/libsrc/osic1p/osic1p.inc +++ b/libsrc/osic1p/osic1p.inc @@ -2,6 +2,7 @@ SCRNBASE := $D000 ; Base of video RAM INPUTC := $FD00 ; Input character from keyboard RESET := $FF00 ; Reset address, show boot prompt +KBD := $DF00 ; Polled keyboard register ; Other definitions VIDEORAMSIZE = $0400 ; Size of C1P video RAM (1 kB) From 222668c016897eea23df781ec619bc288be83502 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sat, 21 Feb 2015 20:24:58 +0100 Subject: [PATCH 02/14] Implemented a one-character buffer for kbhit() and cgetc(). If kbhit() detects that a key is pressed, it fetches and buffers the character. If cgetc() detects a buffered character, this one is returned instead of fetching one with the PROM routine. --- cfg/osic1p.cfg | 2 +- libsrc/osic1p/cgetc.s | 19 ++++++++++++++++++- libsrc/osic1p/extzp.inc | 2 +- libsrc/osic1p/extzp.s | 5 +++-- libsrc/osic1p/kbhit.s | 5 ++++- 5 files changed, 27 insertions(+), 6 deletions(-) diff --git a/cfg/osic1p.cfg b/cfg/osic1p.cfg index 10ce827fd..4771639a6 100644 --- a/cfg/osic1p.cfg +++ b/cfg/osic1p.cfg @@ -7,7 +7,7 @@ SYMBOLS { } MEMORY { # for size of ZP see runtime/zeropage.s and c1p/extzp.s - ZP: file = "", define = yes, start = $0002, size = $001A + $0004; + ZP: file = "", define = yes, start = $0002, size = $001A + $0005; RAM: file = %O, define = yes, start = %S, size = __HIMEM__ - __STACKSIZE__ - %S; } SEGMENTS { diff --git a/libsrc/osic1p/cgetc.s b/libsrc/osic1p/cgetc.s index 3c9dd4381..82857f4c6 100644 --- a/libsrc/osic1p/cgetc.s +++ b/libsrc/osic1p/cgetc.s @@ -1,6 +1,8 @@ ; ; char cgetc (void); ; + + .constructor initcgetc .export _cgetc .import cursor @@ -8,8 +10,21 @@ .include "extzp.inc" .include "zeropage.inc" +; Initialize one-character buffer that is filled by kbhit() +initcgetc: + lda #$00 + sta CHARBUF ; No character in buffer initially + rts + ; Input routine from 65V PROM MONITOR, show cursor if enabled _cgetc: + lda CHARBUF ; character in buffer available? + beq nobuffer + tax ; save character in X + lda #$00 + sta CHARBUF ; empty buffer + jmp restorex ; restore X and return +nobuffer: lda cursor ; show cursor? beq nocursor ldy CURS_X @@ -25,7 +40,9 @@ nocursor: lda tmp1 ; fetch saved character ldy CURS_X sta (SCREEN_PTR),y ; store at cursor position + +restorex: txa ; restore saved character from X - ldx #$00 ; high byte of int return value done: + ldx #$00 ; high byte of int return value rts diff --git a/libsrc/osic1p/extzp.inc b/libsrc/osic1p/extzp.inc index 06498d1a6..5e85e0b74 100644 --- a/libsrc/osic1p/extzp.inc +++ b/libsrc/osic1p/extzp.inc @@ -4,4 +4,4 @@ ; ------------------------------------------------------------------------ - .globalzp CURS_X, CURS_Y, SCREEN_PTR + .globalzp CURS_X, CURS_Y, SCREEN_PTR, CHARBUF diff --git a/libsrc/osic1p/extzp.s b/libsrc/osic1p/extzp.s index 7dc8e3a53..b3bdaa0b7 100644 --- a/libsrc/osic1p/extzp.s +++ b/libsrc/osic1p/extzp.s @@ -14,6 +14,7 @@ CURS_X: .res 1 CURS_Y: .res 1 SCREEN_PTR: .res 2 +CHARBUF: .res 1 -; size 4 -; Adjust size of this segment in osic1p.cfg if the size changes +; size 5 +; Adjust size of the ZP segment in osic1p.cfg if the size changes diff --git a/libsrc/osic1p/kbhit.s b/libsrc/osic1p/kbhit.s index c064fec47..67f3c86ed 100644 --- a/libsrc/osic1p/kbhit.s +++ b/libsrc/osic1p/kbhit.s @@ -19,10 +19,13 @@ scan: rol a ; Rotate row select to next bit position cmp #$FF ; Done? bne scan ; If not, continue - ldx #$00 ; High byte of return is always zero lda #$00 ; Return false + tax ; High byte of return is also zero + sta CHARBUF ; No character in buffer rts keypressed: + jsr INPUTC ; Get input character in A + sta CHARBUF ; Save in buffer ldx #$00 ; High byte of return is always zero lda #$01 ; Return true rts From 1b9aa7c6f3d7cb253d5f28a8ba68b924d796e7e6 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sat, 21 Feb 2015 20:53:16 +0100 Subject: [PATCH 03/14] Use character constants where appropriate. --- libsrc/osic1p/cclear.s | 2 +- libsrc/osic1p/clrscr.s | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) diff --git a/libsrc/osic1p/cclear.s b/libsrc/osic1p/cclear.s index 4e18c70a3..2036c38e0 100644 --- a/libsrc/osic1p/cclear.s +++ b/libsrc/osic1p/cclear.s @@ -22,7 +22,7 @@ _cclear: cmp #0 ; Is the length zero? beq L9 ; Jump if done sta tmp1 -L1: lda #$20 ; Blank - screen code +L1: lda #' ' jsr cputdirect ; Direct output dec tmp1 bne L1 diff --git a/libsrc/osic1p/clrscr.s b/libsrc/osic1p/clrscr.s index ee32bf969..db8da6912 100644 --- a/libsrc/osic1p/clrscr.s +++ b/libsrc/osic1p/clrscr.s @@ -12,7 +12,7 @@ BANKS = VIDEORAMSIZE / $100 _clrscr: - lda #$20 ; ' ' + lda #' ' ldy #BANKS ldx #$00 staloc: From 7a975fa182134b7f1c1bb026cae4900e458bce8a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sat, 21 Feb 2015 22:52:01 +0100 Subject: [PATCH 04/14] Mask control keys from first keyboard scan row. --- libsrc/osic1p/kbhit.s | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) diff --git a/libsrc/osic1p/kbhit.s b/libsrc/osic1p/kbhit.s index 67f3c86ed..b616b4a3f 100644 --- a/libsrc/osic1p/kbhit.s +++ b/libsrc/osic1p/kbhit.s @@ -1,19 +1,35 @@ ; ; unsigned char kbhit (void); +; +; The method to detect a pressed key is based on the documentation in +; "Section 3 Programmed Key Functions" in "The Challenger Character Graphics +; Reference Manual" +; We only want to return true for characters that can be returned by cgetc(), +; but not for keys like or . Therefore a special handling is +; needed for the first row. This is implemented by a bit mask that is stored +; in tmp1 and that is set to zero after the first round. ; .export _kbhit .include "osic1p.inc" + .include "extzp.inc" + .include "zeropage.inc" _kbhit: - lda #%11111110 ; Select first keyboard row + lda #%11011111 ; Mask for only checking the column for the + sta tmp1 ; ESC key in the first keyboard row. + + lda #%11111110 ; Mask for first keyboard row scan: sta KBD ; Select keyboard row tax ; Save A lda KBD ; Read keyboard columns - ora #$01 ; Mask out lsb (Shift Lock), since we ignore it + ora tmp1 ; Mask out uninteresting keys (only relevant in + ; first row) cmp #$FF ; No keys pressed? bne keypressed + lda #$00 ; For remaining rows no keys masked + sta tmp1 txa ; Restore A sec ; Want to shift in ones rol a ; Rotate row select to next bit position From a7dabcda4786bc8db44a7d47b4702ed55ea92fec Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sun, 22 Feb 2015 16:27:13 +0100 Subject: [PATCH 05/14] Define the screen dimension defines in a more meaningful way. --- libsrc/osic1p/_scrsize.s | 4 ++-- libsrc/osic1p/cputc.s | 2 +- libsrc/osic1p/osic1p.inc | 4 ++-- 3 files changed, 5 insertions(+), 5 deletions(-) diff --git a/libsrc/osic1p/_scrsize.s b/libsrc/osic1p/_scrsize.s index 337fe1ee7..b0241c547 100644 --- a/libsrc/osic1p/_scrsize.s +++ b/libsrc/osic1p/_scrsize.s @@ -13,7 +13,7 @@ .include "osic1p.inc" .proc screensize - ldx #(SCR_LINELEN + 1) - ldy #(SCR_HEIGHT + 1) + ldx #SCR_LINELEN + ldy #SCR_HEIGHT rts .endproc diff --git a/libsrc/osic1p/cputc.s b/libsrc/osic1p/cputc.s index 47969df15..9e6ae3253 100644 --- a/libsrc/osic1p/cputc.s +++ b/libsrc/osic1p/cputc.s @@ -35,7 +35,7 @@ cputdirect: ; Advance cursor position advance: - cpy #SCR_LINELEN ; xsize-1 + cpy #(SCR_LINELEN - 1) bne L3 jsr newline ; new line ldy #$FF ; + cr diff --git a/libsrc/osic1p/osic1p.inc b/libsrc/osic1p/osic1p.inc index e95db637e..f49bc122b 100644 --- a/libsrc/osic1p/osic1p.inc +++ b/libsrc/osic1p/osic1p.inc @@ -6,5 +6,5 @@ KBD := $DF00 ; Polled keyboard register ; Other definitions VIDEORAMSIZE = $0400 ; Size of C1P video RAM (1 kB) -SCR_LINELEN = $18 ; screen width - 1 -SCR_HEIGHT = $18 ; screen height - 1 +SCR_LINELEN = $19 ; screen width +SCR_HEIGHT = $19 ; screen height From c3b014560eb291a18b28a70e9516abb49dae34eb Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sun, 22 Feb 2015 19:55:19 +0100 Subject: [PATCH 06/14] Consistent naming for screen dimension constants. --- libsrc/osic1p/_scrsize.s | 2 +- libsrc/osic1p/cputc.s | 2 +- libsrc/osic1p/osic1p.inc | 4 ++-- 3 files changed, 4 insertions(+), 4 deletions(-) diff --git a/libsrc/osic1p/_scrsize.s b/libsrc/osic1p/_scrsize.s index b0241c547..be07234b4 100644 --- a/libsrc/osic1p/_scrsize.s +++ b/libsrc/osic1p/_scrsize.s @@ -13,7 +13,7 @@ .include "osic1p.inc" .proc screensize - ldx #SCR_LINELEN + ldx #SCR_WIDTH ldy #SCR_HEIGHT rts .endproc diff --git a/libsrc/osic1p/cputc.s b/libsrc/osic1p/cputc.s index 9e6ae3253..e986daad1 100644 --- a/libsrc/osic1p/cputc.s +++ b/libsrc/osic1p/cputc.s @@ -35,7 +35,7 @@ cputdirect: ; Advance cursor position advance: - cpy #(SCR_LINELEN - 1) + cpy #(SCR_WIDTH - 1) bne L3 jsr newline ; new line ldy #$FF ; + cr diff --git a/libsrc/osic1p/osic1p.inc b/libsrc/osic1p/osic1p.inc index f49bc122b..b5d48f32e 100644 --- a/libsrc/osic1p/osic1p.inc +++ b/libsrc/osic1p/osic1p.inc @@ -6,5 +6,5 @@ KBD := $DF00 ; Polled keyboard register ; Other definitions VIDEORAMSIZE = $0400 ; Size of C1P video RAM (1 kB) -SCR_LINELEN = $19 ; screen width -SCR_HEIGHT = $19 ; screen height +SCR_WIDTH = $19 ; Screen width +SCR_HEIGHT = $19 ; Screen height From 73ae95bd1c11ccfca9f98d6941ee2600d901c033 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Mon, 23 Feb 2015 19:06:22 +0100 Subject: [PATCH 07/14] Clear bottom line when scrolling. Fix provided by Jeff Tranter. --- libsrc/osic1p/cputc.s | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/libsrc/osic1p/cputc.s b/libsrc/osic1p/cputc.s index 47969df15..058964573 100644 --- a/libsrc/osic1p/cputc.s +++ b/libsrc/osic1p/cputc.s @@ -58,6 +58,12 @@ scroll: lda SCRNBASE+$00A5,x sta SCRNBASE+$0285,x inx bne scroll +bottom: + lda #' ' ; Clear bottom line of screen + sta SCRNBASE+$0385,x + inx + cpx #SCR_LINELEN+1 + bne bottom plot: ldy CURS_Y lda ScrLo,y From 83f16425c9d2dd2d84b4f6932826538e8b7bd01a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Mon, 23 Feb 2015 19:17:27 +0100 Subject: [PATCH 08/14] Loading A once before the loop is sufficient. --- libsrc/osic1p/cputc.s | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/libsrc/osic1p/cputc.s b/libsrc/osic1p/cputc.s index 058964573..4e16e92df 100644 --- a/libsrc/osic1p/cputc.s +++ b/libsrc/osic1p/cputc.s @@ -58,8 +58,9 @@ scroll: lda SCRNBASE+$00A5,x sta SCRNBASE+$0285,x inx bne scroll -bottom: + lda #' ' ; Clear bottom line of screen +bottom: sta SCRNBASE+$0385,x inx cpx #SCR_LINELEN+1 From 7c4f96de817d2c63dbb41d6a72c12db1e1d2c5be Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Mon, 23 Feb 2015 20:28:05 +0100 Subject: [PATCH 09/14] kbhit implemented, note about limitation removed. --- doc/osi.sgml | 4 ---- 1 file changed, 4 deletions(-) diff --git a/doc/osi.sgml b/doc/osi.sgml index ce2ee4836..e360daf7a 100644 --- a/doc/osi.sgml +++ b/doc/osi.sgml @@ -137,10 +137,6 @@ There are no loadable drivers available. Limitations

-conio implementation

- -The conio implementation is complete except for a kbhit() function. - stdio implementation

There is no support for stdio at the moment. From 8deeb2c59e50c4221dcf588e9670b63f4e26e729 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Mon, 23 Feb 2015 23:07:00 +0100 Subject: [PATCH 10/14] Match screen dimensions as in BASIC. Experiments show that under BASIC by default 25 rows by 24 columns are used. 24 columns is also the width that is fully displayed on a real C1P on the monitor. conio now matches that now. --- libsrc/osic1p/osic1p.inc | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libsrc/osic1p/osic1p.inc b/libsrc/osic1p/osic1p.inc index b5d48f32e..d209b258d 100644 --- a/libsrc/osic1p/osic1p.inc +++ b/libsrc/osic1p/osic1p.inc @@ -6,5 +6,5 @@ KBD := $DF00 ; Polled keyboard register ; Other definitions VIDEORAMSIZE = $0400 ; Size of C1P video RAM (1 kB) -SCR_WIDTH = $19 ; Screen width +SCR_WIDTH = $18 ; Screen width SCR_HEIGHT = $19 ; Screen height From 52865410d2269e074bb7c41683d023f875b69f19 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Wed, 25 Feb 2015 23:48:57 +0100 Subject: [PATCH 11/14] Fix wrong expression for top of C stack. Thanks to Greg King for this fix. --- libsrc/osic1p/crt0.s | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/libsrc/osic1p/crt0.s b/libsrc/osic1p/crt0.s index 657ee2743..62342c206 100644 --- a/libsrc/osic1p/crt0.s +++ b/libsrc/osic1p/crt0.s @@ -9,6 +9,7 @@ .export __STARTUP__ : absolute = 1 ; Mark as startup .import __RAM_START__, __RAM_SIZE__ ; Linker generated +.import __STACKSIZE__ .import zerobss, initlib, donelib @@ -31,9 +32,9 @@ _init: ldx #$FF ; Initialize stack pointer to $01FF ; --------------------------------------------------------------------------- ; Set cc65 argument stack pointer - lda #<(__RAM_START__ + __RAM_SIZE__) + lda #<(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__) sta sp - lda #>(__RAM_START__ + __RAM_SIZE__) + lda #>(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__) sta sp+1 ; --------------------------------------------------------------------------- From 4737ad93d5cc8882675eaa12533937b88763ed29 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Wed, 25 Feb 2015 23:50:02 +0100 Subject: [PATCH 12/14] Temporarily undo the 25 line change. This is not yet consistent with the first visible character on the screen. --- libsrc/osic1p/osic1p.inc | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libsrc/osic1p/osic1p.inc b/libsrc/osic1p/osic1p.inc index d209b258d..232c471aa 100644 --- a/libsrc/osic1p/osic1p.inc +++ b/libsrc/osic1p/osic1p.inc @@ -7,4 +7,4 @@ KBD := $DF00 ; Polled keyboard register ; Other definitions VIDEORAMSIZE = $0400 ; Size of C1P video RAM (1 kB) SCR_WIDTH = $18 ; Screen width -SCR_HEIGHT = $19 ; Screen height +SCR_HEIGHT = $18 ; Screen height From 1d6aa84d9a8521757c4c067feb059fc2d38109d7 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sat, 28 Feb 2015 20:29:08 +0100 Subject: [PATCH 13/14] Optimization by replacing absolute with relative jump. --- libsrc/osic1p/cgetc.s | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libsrc/osic1p/cgetc.s b/libsrc/osic1p/cgetc.s index 82857f4c6..0c7c69488 100644 --- a/libsrc/osic1p/cgetc.s +++ b/libsrc/osic1p/cgetc.s @@ -23,7 +23,7 @@ _cgetc: tax ; save character in X lda #$00 sta CHARBUF ; empty buffer - jmp restorex ; restore X and return + beq restorex ; restore X and return nobuffer: lda cursor ; show cursor? beq nocursor From 981ab10fac12c7f4ba18d8a08eb1b949eea7894a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Stephan=20M=C3=BChlstrasser?= Date: Sun, 1 Mar 2015 20:53:57 +0100 Subject: [PATCH 14/14] Replaced magic numbers with symbolic constants. --- libsrc/osic1p/cputc.s | 24 ++++++++++++++---------- 1 file changed, 14 insertions(+), 10 deletions(-) diff --git a/libsrc/osic1p/cputc.s b/libsrc/osic1p/cputc.s index e4416682a..2baada465 100644 --- a/libsrc/osic1p/cputc.s +++ b/libsrc/osic1p/cputc.s @@ -12,6 +12,10 @@ .include "osic1p.inc" .include "extzp.inc" +FIRSTVISC = $85 ; Offset of first visible character in video RAM +LINEDIST = $20 ; Offset in video RAM between two lines +BLOCKSIZE = $100 ; Size of block to scroll + _cputcxy: pha ; Save C jsr popa ; Get Y @@ -37,7 +41,7 @@ cputdirect: advance: cpy #(SCR_WIDTH - 1) bne L3 - jsr newline ; new line + jsr newline ; New line ldy #$FF ; + cr L3: iny sty CURS_X @@ -46,22 +50,22 @@ L3: iny newline: inc CURS_Y lda CURS_Y - cmp #SCR_HEIGHT ; screen height + cmp #SCR_HEIGHT ; Screen height bne plot - dec CURS_Y ; bottom of screen reached, scroll + dec CURS_Y ; Bottom of screen reached, scroll ldx #0 -scroll: lda SCRNBASE+$00A5,x - sta SCRNBASE+$0085,x - lda SCRNBASE+$01A5,x - sta SCRNBASE+$0185,x - lda SCRNBASE+$02A5,x - sta SCRNBASE+$0285,x +scroll: +.repeat 3, I ; Scroll screen in three blocks of size + ; BLOCKSIZE + lda SCRNBASE+(I*BLOCKSIZE)+FIRSTVISC+LINEDIST,x + sta SCRNBASE+(I*BLOCKSIZE)+FIRSTVISC,x +.endrepeat inx bne scroll lda #' ' ; Clear bottom line of screen bottom: - sta SCRNBASE+$0385,x + sta SCRNBASE+(3*BLOCKSIZE)+FIRSTVISC,x inx cpx #SCR_WIDTH bne bottom