I'm trying to figure out how to create a VIC20 program with ca65 that can run on a VIC20 + 16K expansion.
The memory layout would be something like:
- 0x1200: BASIC START LINE
- 0x2000: PROGRAM
I'm trying to create this with result the ld65 linker, but have had to search quite a bit to get my answer.
As always RTFM is the right answer, but it can take time to digest and understand the context.
Here's my findings...
Just a simple 'Hello' program....
I'm using 2 segments here BASIC and CODE.
.segment "BASIC"
.word load
load: .word @end
.word 2020
.byte $9e
.byte .lobyte(main/1000 .mod 10) + $30
.byte .lobyte(main/100 .mod 10) + $30
.byte .lobyte(main/10 .mod 10) + $30
.byte .lobyte(main/1 .mod 10) + $30
.byte 0
@end: .word 0
.segment "CODE"
main: ldx #0
: lda text,x
beq end
jsr $ffd2
inx
clc
bcc :-
end: rts
text:
.asciiz "HELLO"
Compile to main.o:
ca65 -I. --cpu 6502 main.s
FEATURES {
STARTADDRESS: default = $11FF;
}
MEMORY {
ZP: start = $0000, size = $0100, type = rw;
RAM0: start = $0400, size = $0C00, type = rw;
RAM: start = $11FF, size = $0E00, type = rw;
RAM1: start = $2000, size = $4000, type = rw;
}
SEGMENTS {
ZEROPAGE: load = ZP, type = zp, optional = yes;
BASIC: load = RAM, type = rw, optional = no ;
CODE: load = RAM, type = rw, optional = yes;
DATA: load = RAM, type = rw, optional = yes;
BSS: load = RAM, type = bss, optional = yes;
}
All segments are in RAM.
Link:
ld65 -Ln hello-1.lbl -m hello-1.map -C vic20-16k-asm-1.cfg -o hello-1.prg main.o
Result:
00000000: 0112 0b12 e407 9e34 3632 3100 0000 a200 .......4621.....
00000010: bd1c 12f0 0720 d2ff e818 90f4 6048 454c ..... ......`HEL
00000020: 4c4f 00 LO.
FEATURES {
STARTADDRESS: default = $11FF;
}
MEMORY {
ZP: start = $0000, size = $0100, type = rw;
RAM0: start = $0400, size = $0C00, type = rw;
RAM: start = $11FF, size = $0E00, type = rw;
RAM1: start = $2000, size = $4000, type = rw;
}
SEGMENTS {
ZEROPAGE: load = ZP, type = zp, optional = yes;
BASIC: load = RAM, type = rw, optional = no ;
CODE: load = RAM1, type = rw, optional = yes;
DATA: load = RAM1, type = rw, optional = yes;
BSS: load = RAM1, type = bss, optional = yes;
}
BASIC segment is in RAM, CODE segment is in RAM1.
Link:
ld65 -Ln hello-2.lbl -m hello-2.map -C vic20-16k-asm-2.cfg -o hello-2.prg main.o
Result:
00000000: 0112 0b12 e407 9e38 3139 3200 0000 a200 .......8192.....
00000010: bd0f 20f0 0720 d2ff e818 90f4 6048 454c .. .. ......`HEL
00000020: 4c4f 00 LO.
The result is the same... I had hoped that the linker would grow the program so that the code in the 'CODE' segment would live at location 0x2000.
00000000: 0112 0b12 e407 9e38 3139 3200 0000 0000
00000010: 0000 0000 0000 0000 0000 0000 0000 0000
...
xxxxxxxx: a200 bd0f 20f0 0720 d2ff e818 90f4 454c <<-- Pointing at 0x02000
...
FEATURES {
STARTADDRESS: default = $11FF;
}
MEMORY {
ZP: start = $0000, size = $0100, type = rw;
RAM0: start = $0400, size = $0C00, type = rw;
RAM: start = $11FF, size = $4E01, type = rw, fill = yes, fillVal = $00;
}
SEGMENTS {
ZEROPAGE: load = ZP, type = zp, optional = yes;
BASIC: load = RAM, type = rw, optional = no ;
DATA: load = RAM, type = rw, optional = yes;
BSS: load = RAM, type = bss, optional = yes;
CODE: load = RAM, type = overwrite, optional = yes, start = $2000;
}
BASIC and CODE segment are in RAM, but CODE is placed at a fixed offset and RAM is filled.
Link:
ld65 -Ln hello-3.lbl -m hello-3.map -C vic20-16k-asm-3.cfg -o hello-3.prg main.o
Result:
00000000: 0112 0b12 e407 9e38 3139 3200 0000 0000 .......8192.....
00000010: 0000 0000 0000 0000 0000 0000 0000 0000 ................
...
00000df0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
00000e00: 00a2 00bd 0f20 f007 20d2 ffe8 1890 f460 ..... .. ......`
00000e10: 4845 4c4c 4f00 0000 0000 0000 0000 0000 HELLO...........
00000e20: 0000 0000 0000 0000 0000 0000 0000 0000 ................
...
00004de0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
00004df0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
Success!
What happens here is that the configuration instructs the linker to fill RAM with zeros, starting from 11FF to 6000. The CODE segment is placed inside RAM, but at a specific location, 2000, overwriting a portion of RAM.
Here are some pointers to the relevant documentation:
The method with fill/overwrite works, but generates a huge file. A better approach would be to split the application in two parts and use a loader.
The linker has support for generating the right data to do this, via the define and file options.
Then a loader (tape loader) needs to be created that will be placed in the BASIC segment and that loads the other part of the program to the right location.
That's another project.