See Also: part 1, part 2, part 3, part 4, part 5 and more to come…
One thing that bugs me about this series is the title. We are not actually hacking the PRINT command. Instead, we are changing the CHROUT character output routine. Close enough, I guess, since PRINT uses this routine…
Now let’s take a tangent to my “Attract Screen” series of posts from a few years ago, specifically the final installment:
Color BASIC Attract Screen – part 6
In that series, I was playing with ways to create the classic CoCo attract screens with the colored blocks that went around the screen:
In that article, Robert Gault had pointed me to this specific “WAR!” program sold by Radio Shack. It had embedded stuff in a PRINT command. I dissected those lines. I found some contained embedded CHR$ graphics characters, and another contained an assembly language routine. Very cool BASIC hacking.
In BASIC, there is an “uncrunch” routine that converts tokenized BASIC to full-text output via the LIST command. LIST will not have a good time LISTing such programs. In BASIC, any byte with the high-bit set (128-255 value) is seen as a token and the tokenizer takes over to convert that one (or two) byte sequence to a word.
Instead of showing a CHR$(128) graphics block, LIST will show the token keyword that is 128.
This led me to wonder if I could create a patch for BASIC that would allow it to properly LIST lines with embedded characters like this. And I did. Here is the test program I wrote back then and completely forgot about until a few days ago.
UPPERCASE stuff is taken from the Color BASIC ROM code. You will see I had to clone most of the token parsing code in my program so I could modify its behavior.
* lwasm uncrunch.asm -fbasic -ouncrunch.bas --map
*
* 0.00 2022-07-04 allenh - initial klunky version.
*
* Allow LIST to display graphics characters inside of quoted strings.
RVEC24 equ $1A6 UNCRUNCH BASIC LINE RAM hook
COMVEC EQU $0120 Some BASIC locations we need.
LINBUF EQU $02DC
SKP2 EQU $8C
LBUFMX EQU 250
org $3f00
init
lda RVEC24 get op code
sta savedrvec save it
ldx RVEC24+1 get address
stx savedrvec+1 save it
lda #$7e op code for JMP
sta RVEC24 store it in RAM hook
ldx #newcode address of new code
stx RVEC24+1 store it in RAM hook
rts done
newcode
* UNCRUNCH A LINE INTO BASIC'S LINE INPUT BUFFER
LB7C2
clr AREWEQUOTED
*JSR >RVEC24 HOOK INTO RAM
LEAS 2,S Remove JSR from stack
LEAX 4,X MOVE POINTER PAST ADDRESS OF NEXT LINE AND LINE NUMBER
LDY #LINBUF+1 UNCRUNCH LINE INTO LINE INPUT BUFFER
LB7CB
LDA ,X+ GET A CHARACTER
LBEQ LB820 BRANCH IF END OF LINE
* Check for quote/unquote
cmpa #34 Is A a quote character?
bne quotedone
togglequote
tst AREWEQUOTED
bne quoteoff
quoteon
inc AREWEQUOTED
bra quotedone
quoteoff
clr AREWEQUOTED Toggle quote mode off.
quotedone
tst AREWEQUOTED
beq notquoted
quoted
* If we are quoted, just store whatever it is.
lda -1,x
CMPY #LINBUF+LBUFMX TEST FOR END OF LINE INPUT BUFFER
BCC LB820 BRANCH IF AT END OF BUFFER
*ANDA #$7F MASK OFF BIT 7
STA ,Y+ * SAVE CHARACTER IN BUFFER AND
CLR ,Y * CLEAR NEXT CHARACTER SLOT IN BUFFER
BRA LB7CB GET ANOTHER CHARACTER
notquoted
lda -1,x
LBMI LB7E6 BRANCH IF IT'S A TOKEN
CMPA #': CHECK FOR END OF SUB LINE
BNE LB7E2 BRNCH IF NOT END OF SUB LINE
LDB ,X GET CHARACTER FOLLOWING COLON
CMPB #$84 TOKEN FOR ELSE?
BEQ LB7CB YES - DON'T PUT IT IN BUFFER
CMPB #$83 TOKEN FOR REMARK?
BEQ LB7CB YES - DON'T PUT IT IN BUFFER
FCB SKP2 SKIP TWO BYTES
LB7E0
LDA #'! EXCLAMATION POINT
LB7E2
BSR LB814 PUT CHARACTER IN BUFFER
BRA LB7CB GET ANOTHER CHARACTER
* UNCRUNCH A TOKEN
LB7E6
LDU #COMVEC-10 FIRST DO COMMANDS
CMPA #$FF CHECK FOR SECONDARY TOKEN
BNE LB7F1 BRANCH IF NON SECONDARY TOKEN
LDA ,X+ GET SECONDARY TOKEN
LEAU 5,U BUMP IT UP TO SECONDARY FUNCTIONS
LB7F1
ANDA #$7F MASK OFF BIT 7 OF TOKEN
LB7F3
LEAU 10,U MOVE TO NEXT COMMAND TABLE
TST ,U IS THIS TABLE ENABLED?
BEQ LB7E0 NO - ILLEGAL TOKEN
LB7F9
SUBA ,U SUBTRACT THE NUMBER OF TOKENS FROM THE CURRENT TOKEN NUMBER
BPL LB7F3 BRANCH IF TOKEN NOT IN THIS TABLE
ADDA ,U RESTORE TOKEN NUMBER RELATIVE TO THIS TABLE
LDU 1,U POINT U TO COMMAND DICTIONARY TABLE
LB801
DECA DECREMENT TOKEN NUMBER
BMI LB80A BRANCH IF THIS IS THE CORRECT TOKEN
* SKIP THROUGH DICTIONARY TABLE TO START OF NEXT TOKEN
LB804
TST ,U+ GRAB A BYTE
BPL LB804 BRANCH IF BIT 7 NOT SET
BRA LB801 GO SEE IF THIS IS THE CORRECT TOKEN
LB80A
LDA ,U GET A CHARACTER FROM DICTIONARY TABLE
BSR LB814 PUT CHARACTER IN BUFFER
TST ,U+ CHECK FOR START OF NEXT TOKEN
BPL LB80A BRANCH IF NOT DONE WITH THIS TOKEN
BRA LB7CB GO GET ANOTHER CHARACTER
LB814
CMPY #LINBUF+LBUFMX TEST FOR END OF LINE INPUT BUFFER
BCC LB820 BRANCH IF AT END OF BUFFER
ANDA #$7F MASK OFF BIT 7
STA ,Y+ * SAVE CHARACTER IN BUFFER AND
CLR ,Y * CLEAR NEXT CHARACTER SLOT IN BUFFER
LB820
RTS
* Unused at the moment.
savedrvec rmb 3 call regular RAM hook
rts just in case...
AREWEQUOTED rmb 1
end $3f00
And here is a BASIC loader for this routine:
10 READ A,B
20 IF A=-1 THEN 70
30 FOR C = A TO B
40 READ D:POKE C,D
50 NEXT C
60 GOTO 10
70 EXEC 16128
80 DATA 16128,16290,182,1,166,183,63,163,190,1,167,191,63,164,134,126,183,1,166,142,63,24,191,1,167,57,127,63,167,50,98,48,4,16,142,2,221,166,128,16,39,0,121,129,34,38,13,125,63,167,38,5,124,63,167,32,3,127,63,167,125,63,167,39,14,166,31,16,140
90 DATA 3,214,36,91,167,160,111,164,32,214,166,31,16,43,0,21,129,58,38,13,230,132,193,132,39,198,193,131,39,194,140,134,33,141,48,32,187,206,1,22,129,255,38,4,166,128,51,69,132,127,51,74,109,196,39,231,160,196,42,246,171,196,238,65,74,43,6,109
100 DATA 192,42,252,32,247,166,196,141,6,109,192,42,248,32,141,16,140,3,214,36,6,132,127,167,160,111,164,57,16294,16294,57,-1,-1
If you RUN that, then install it using EXEC &H3F00, you can now LIST and see characters embedded in strings:
I bring this up now because 1) I forgot to post about it back in 2022, and 2) because I think I want to do something similar with my cursor movement PRINT patch. Ideally, you should be able to LIST a program and see the original characters in it, and just have them move around when the program is running and PRINTs those characters. This matches how the VIC-20 worked with embedded characters inside a PRINT:
Screenshot from the Javascript Vic20 Emuator: https://www.mdawson.net/vic20chrome/vic20.php
Since I do not remember how this worked, I thought we could go through this program and see what it does.
* lwasm uncrunch.asm -fbasic -ouncrunch.bas --map
*
* 0.00 2022-07-04 allenh - initial klunky version.
*
* Allow LIST to display graphics characters inside of quoted strings.
RVEC24 equ $1A6 UNCRUNCH BASIC LINE RAM hook
COMVEC EQU $0120 Some BASIC locations we need.
LINBUF EQU $02DC
SKP2 EQU $8C
LBUFMX EQU 250
The start is very similar to the consmove.asm code presented in the first parts of this series. RVEC24 is the RAM hook for the UNCRUNCH routine. Microsoft made this available so future BASICs (like Extended, Disk, etc.) could support LISTING their new tokens, I assume.
LINBUF is the BASIC input buffer. This is where things go when you are typing at the OK prompt.
SKP2 is a thing Microsoft used in the ROM as a shortcut to skip two bytes. It is $8C, which is “CMPX #”. It seems they could place that in the code and it would be an “Load X” then whatever two bytes were behind it, allowing a shortcut to branch over those two bytes. Can anyone explain this better (or more accurately, or just accurately)? Leave a comment if you can.
And lastly was a define that was the max size of the line buffer – 250 bytes. I have not looking into why it was 250 instead of, say, 255, but this is as many characters as you can type before input stops and you can only backspace or press ENTER.
org $3f00
init
lda RVEC24 get op code
sta savedrvec save it
ldx RVEC24+1 get address
stx savedrvec+1 save it
lda #$7e op code for JMP
sta RVEC24 store it in RAM hook
ldx #newcode address of new code
stx RVEC24+1 store it in RAM hook
rts doneThis is the code that installs the RAM hook. It should look close to what I have in the previous routine, just with a different vector.
Now we get to the new code. For this I needed to duplicate some code in the BASIC ROM. The code in UPPERCASE represents code I brought in from the Unravelled disassembly listing. The ROM code has no provision for doing something different if it is a quoted string, so I needed to use the original code, with some extra code around that it turns on or off the detokenizer if we are listing something within quotes.
Things are about to get messy…
newcode
* UNCRUNCH A LINE INTO BASIC'S LINE INPUT BUFFER
LB7C2
clr AREWEQUOTED
*JSR RVEC24 HOOK INTO RAM
LEAS 2,S Remove JSR from stack
LEAX 4,X MOVE POINTER PAST ADDRESS OF NEXT LINE AND LINE NUMBER
LDY #LINBUF+1 UNCRUNCH LINE INTO LINE INPUT BUFFER
LB7CB
LDA ,X+ GET A CHARACTER
LBEQ LB820 BRANCH IF END OF LINEThis code is mostly from the BASIC ROM except for that first “clr”. I reserve a byte of memory to use as a flag for when we are or are not parsing something in quotes. If a quote is seen, that flag is set. It stays set until another quote is seen or the end of line is reached. The commented-out line is in the original ROM and that would be the line that jumps to this hook. I kept the line in for clarity, but it is not used in my function.
Now we have my custom code. If we are not at the end of line, I check for quotes and turn the flag on or off:
* Check for quote/unquote
cmpa #34 Is A a quote character?
bne quotedone
togglequote
tst AREWEQUOTED
bne quoteoff
quoteon
inc AREWEQUOTED
bra quotedone
quoteoff
clr AREWEQUOTED Toggle quote mode off.
quotedone
tst AREWEQUOTED
beq notquotedThis flag will be used in a moment. I clone some of the ROM code that outputs the un-crunched line but will bypass the un-crunch for items within quotes.
quoted
* If we are quoted, just store whatever it is.
lda -1,x
CMPY #LINBUF+LBUFMX TEST FOR END OF LINE INPUT BUFFER
BCC LB820 BRANCH IF AT END OF BUFFER
*ANDA #$7F MASK OFF BIT 7
STA ,Y+ * SAVE CHARACTER IN BUFFER AND
CLR ,Y * CLEAR NEXT CHARACTER SLOT IN BUFFER
BRA LB7CB GET ANOTHER CHARACTERFor quoted items, A is loaded with a character. Then the ROM code runs but I commented out the thing that would mask off bit 7. Tokens have that bit set, but I wanted to leave high-bit bytes alone.
This batch of code is more Color BASIC ROM code I duplicated here after that first “lda” line:
notquoted
lda -1,x
LBMI LB7E6 BRANCH IF IT'S A TOKEN
CMPA #': CHECK FOR END OF SUB LINE
BNE LB7E2 BRNCH IF NOT END OF SUB LINE
LDB ,X GET CHARACTER FOLLOWING COLON
CMPB #$84 TOKEN FOR ELSE?
BEQ LB7CB YES - DON'T PUT IT IN BUFFER
CMPB #$83 TOKEN FOR REMARK?
BEQ LB7CB YES - DON'T PUT IT IN BUFFER
FCB SKP2 SKIP TWO BYTES
LB7E0
LDA #'! EXCLAMATION POINT
LB7E2
BSR LB814 PUT CHARACTER IN BUFFER
BRA LB7CB GET ANOTHER CHARACTER
* UNCRUNCH A TOKEN
LB7E6
LDU #COMVEC-10 FIRST DO COMMANDS
CMPA #$FF CHECK FOR SECONDARY TOKEN
BNE LB7F1 BRANCH IF NON SECONDARY TOKEN
LDA ,X+ GET SECONDARY TOKEN
LEAU 5,U BUMP IT UP TO SECONDARY FUNCTIONS
LB7F1
ANDA #$7F MASK OFF BIT 7 OF TOKEN
LB7F3
LEAU 10,U MOVE TO NEXT COMMAND TABLE
TST ,U IS THIS TABLE ENABLED?
BEQ LB7E0 NO - ILLEGAL TOKEN
LB7F9
SUBA ,U SUBTRACT THE NUMBER OF TOKENS FROM THE CURRENT TOKEN NUMBER
BPL LB7F3 BRANCH IF TOKEN NOT IN THIS TABLE
ADDA ,U RESTORE TOKEN NUMBER RELATIVE TO THIS TABLE
LDU 1,U POINT U TO COMMAND DICTIONARY TABLE
LB801
DECA DECREMENT TOKEN NUMBER
BMI LB80A BRANCH IF THIS IS THE CORRECT TOKEN
* SKIP THROUGH DICTIONARY TABLE TO START OF NEXT TOKEN
LB804
TST ,U+ GRAB A BYTE
BPL LB804 BRANCH IF BIT 7 NOT SET
BRA LB801 GO SEE IF THIS IS THE CORRECT TOKEN
LB80A
LDA ,U GET A CHARACTER FROM DICTIONARY TABLE
BSR LB814 PUT CHARACTER IN BUFFER
TST ,U+ CHECK FOR START OF NEXT TOKEN
BPL LB80A BRANCH IF NOT DONE WITH THIS TOKEN
BRA LB7CB GO GET ANOTHER CHARACTER
LB814
CMPY #LINBUF+LBUFMX TEST FOR END OF LINE INPUT BUFFER
BCC LB820 BRANCH IF AT END OF BUFFER
ANDA #$7F MASK OFF BIT 7
STA ,Y+ * SAVE CHARACTER IN BUFFER AND
CLR ,Y * CLEAR NEXT CHARACTER SLOT IN BUFFER
LB820
RTS
* Unused at the moment.
savedrvec rmb 3 call regular RAM hook
rts just in case...
AREWEQUOTED rmb 1
end $3f00Looking at this now, I expect I had to duplicate all this ROM code because I need to stay in the un-crunch loop for the complete line. If you look at the ROM code and see a better approach, please leave a comment.
I also think this may be wrong, since I do not see it calling the original vector when complete. This may not work for tokens added by Extended or Disk BASIC. We will have to fix that at some point.
For now, I will just leave this here as an example, and then figure out how I could do something similar with my cursor move codes so they LIST showing the codes, and only move around the screen when PRINTing them.
This should be interesting… The way I patched CHROUT, it has no idea if the character is from LIST or PRINT. I will have to figure out how to solve that, next.
Until then…
