Commodore PET - Peek Inquiry

[evildead9000]

Hello All!

 

I've recently purchased a Commodore Pet and it's awesome! I'm loving all the Petscii and text games!!

 

I've also decided to code my own Petscii game. I have some knowledge of basic, and I've been working to learn more.

However, I'm having a bit of difficultly with Peek, which brings me here.

 

I'm hoping someone could shed some light. Right now I'm just testing out things to see how things work.

 

Here's my issue: I have an A as the first character on screen. I can peek that location and print the character on-screen below it. It's a 1 according to the PETSCII character chart. However, when I change the A into an inverted/reverse A, the number 32 is printed on screen. All inverted characters are printing a 32, which is a space.

 

How can I peek a location to verify if a specific inverted/reverse character is there?

 

I hope I explained the issue correctly and please correct me if I am incorrect! Any guidance/info would be greatly appreciated.

 

Thank You!

 

Long live the Commodore! (all of them)

 

[brfennpocock]

If I understand the problem correctly …

 

PETSCII isn't used for screen display codes; I believe the PET will use a similar (same?) code set as the VIC-20 / C=64 / C=128, seen in http://sta.c64.org/cbm64scr.html — It's like PETSCII with the 3° group of 32 chars swapped into the 1° position. A in PETSCII = $41, A in screen codes = $01.

 

A reverse char is the same as its normal char & $80 (+128), so where A is $01, reverse A is $81; π is $5e, reverse π is $de.

 

To get the PETSCII for a non-reverse char, and/or a flag to indicate if a char is reversed,

REM screen address
AD% = (screen code base RAM address) + Y% * (screen width) + X%

REM is it reversed? 0 = false, 128 = true.
RV% = PEEK(AD%) AND 128

REM convert screen code to PETSCII
SC% = PEEK(AD%) AND 127
PC% = SC%:IF SC% < 32 THEN PC%=SC%+64

 

 

Edited September 25, 2019 by brfennpocock

[evildead9000]

Thanks for the quick response!

 

Your explanation clarifies some things. I should be good moving forward. Thanks much!

[carlsson]

And for anyone wondering, the screen base address here is 32768.