PLUS4-CUSTOM-CHARACTERS
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TOOL: 		MERLIN 7501 ASSEMBLER {Commodore PLUS4}
Language: 	Assembler 
Extra:		none
Author: 	Misel Zivanovic
Note:		(C) 27th JULY 2026 - www.lived.ch, mzretro.ch
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This is my 2nd program for the Commodore Plus4.
It is creating a custom character set, which is running in the background {IRQ}
It's just the alphabet {the same i made for the ATARI earlier}, but I will extend it in the future
to also include numbers etc.

Don't let this stop you from making your own character set!


HOW TO USE IT /PROCEED:
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1st START the MERLIN 7501 ASSEMBLER from the disk!

2nd MOVE THE SOURCE CODE START ADDRESS TO: $5000 with this command in direct mode {no line number}: 
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	poke 43,1:poke44,80: poke 20480,0:new
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This will let the default $2100 source code address free for the assembly code
and that is why the start address can be $2100 {com $2100}

Additional info:
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When you save the source code and load it the next time, it will go again to the default address
{$2100} that means that you need to move the start address every time when you assemble the code
to $2100.That is including loading too. 
So first move the source code address, then load a file you need.
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Now back to regular business:

Copy/paste this code below to VICE Plus4 Emulator and assemble it with the ASM command.
It can be started with SYS 8448 and it is running as an interrupt.

You can exit this custom characters at any time by placing the cursor to the LEFT UPPER CORNER and
then press there on SHIFT + Q to exit.

10 com $2100
11 list
12 lda .stat
13 cmp #55
14 beq .nomore
15 bne .runit
16 .nomore
17 rts
18 .runit
19 lda #55
20 sta .stat
100 ;assembler merlin 7501
103 ;c= plus 4 - mzretro-ch
104 ;custom characters
106 ;start with sys 8448
100 ;
110 lda #$00
111 sta $03
112 sta $05
113 lda #$38
114 sta $04
115 lda #$d0 
116 sta $06
117 ldy #$00
118 .loop1
119 lda ($05),y
120 sta ($03),y
122 iny
123 bne .loop1
124 inc $04
125 inc $06
126 lda #$d8
127 cmp $06
128 bne .loop1
129 lda $ff12 ;read status
130 and #251  ;switch 
131 sta $ff12 ;ram characters
132 lda $ff13 ;set
133 and #3
134 ora #56   ;location
135 sta $ff13 ;$3800
136 ;
137 ldx #$00  ;custom chars
138 .more
139 lda .chars,x
140 sta $3808,x ;no @
141 inx
142 cpx #208   ;26 characters
143 bne .more ;
150 sei
151 lda #$61  ;when code is added
152 sta $0314 ;check the low and
153 lda #$21  ;high byte address!
154 sta $0315 ;for the irq start
161 ldy #$00  ;
162 sty $0be8 ;main delay
167 cli
168 rts
169 ;
170 ldy $0be8 ;< irq start address
171 iny       ;  low high byte 
172 sty $0be8
173 ldy $0be8
174 cpy #$10  ;keep y > 1!
175 beq .mychars
176 ;----------
177 lda $0c00 ;press shift+q
178 cmp #81   ;to exit the
189 beq .out  ;interrupt mode!
190 .exit     ;where:
191 jmp $ce0e ;left upper corner!
200 ;         ;in capitol char mode
250 .out      ;exit to basic, no irq
251 lda $ff12 ;read status
252 ora #4    ;set characters
253 sta $ff12 ;back to
254 lda $ff13 ;original
255 and #3    ;rom 
256 ora #$d0  ;source
257 sta $ff13 ;$d000-53248
258 lda #$00
259 sta .stat
260 sei       ;safe exit
261 lda #14   ;no reset needed!
262 sta $0314
263 lda #206
264 sta $0315
265 cli
266 pla
267 tay
268 pla
269 tax
270 pla
271 rti
272 rts
280 ;
300 .mychars
700 ldy #$00    ;reset main delay
701 sty $0be8   ;
702 jmp .exit   ;!mandatory!
703 ;
704 ;2nd run yes/no
705 .stat
706 *da $00
707 ;
800 .chars 
888 ;@ unused
901 ;255,129,129,129,129,129,255,0
900 ;a,b,c,d,e,f
901 *da 30,62,118,230,254,198,198,0
903 *da 240,248,220,254,198,198,252,0
904 *da 62,126,224,192,192,198,254,0
905 *da 248,252,206,198,198,198,254,0
906 *da 62,126,224,252,192,192,254,0
907 *da 62,126,192,252,192,192,192,0
908 ;g,h,i,j,k,l,m
909 *da 28,62,118,224,206,198,254,0
910 *da 54,118,230,198,254,198,198,0
911 *da 12,28,56,48,48,48,48,0
912 *da 6,14,28,24,24,88,120,0
913 *da 206,222,248,240,248,222,206,0
914 *da 48,112,224,192,192,192,252,0
915 *da 54,118,230,238,254,214,198,0
916 ;n,o,p,q,r,s,t
917 *da 54,118,230,246,254,222,206,0
918 *da 60,126,230,198,198,198,254,0
919 *da 252,254,198,198,254,192,192,0
920 *da 60,126,230,198,198,204,254,6
921 *da 252,254,198,198,254,220,206,0
922 *da 62,126,224,252,14,14,252,0
923 *da 254,254,48,48,112,96,96,0
924 ;u,v,w,x,y,z
925 *da 54,118,230,198,198,198,124,0
926 *da 100,230,198,198,110,124,56,0
927 *da 96,230,198,198,214,254,110,0
928 *da 102,230,198,124,108,198,198,0
929 *da 102,230,206,124,48,48,48,0
930 *da 254,206,24,48,48,96,254,0
931 ;chars done
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