Wednesday, June 12, 2024

Strings or Compact directory utility for C64

Directory listings scrolling by too fast?  Want to scan program for string literals?  Here's a stupid (sometimes useful, definitely ugly) utility I created in the monitor for both scenarios.  Compact display and pauses every 24 lines.  Attempts to avoid control characters.

Note: operates on whatever loaded program is in memory, assuming BASIC. 

Shown loading into tape buffer (828), can be relocated. 

This source is an example of parsing a BASIC program.  The beginning of BASIC is grabbed from $2B/2C, and each line of the program is iterated through traveling its linked list ending with a null pointer.  In each line, a string starts with double quotes and ends with double quotes or end of line (zero or nul character).  Multiple strings per lines are possible.   Other memory addresses used include toggling the reverse flag ($C7), setting quote ($D4) and insert ($D8) modes, and counting each time at the left column ($D3).  Outputs string characters via $FFD2, and waits for a key press via $FFE4.

This is a short program that can be studied.  I hope you find it useful and informative.  Enjoy!

Link: strings.prg
Copyright (c) 2024 by David R. Van Wagner

Sunday, June 2, 2024


While adding banking support to my own C64 emulator back in the good 'ole days (April 14, 2020, maybe only good for hunkering down on retro stuff), I also wrote a test program to help verify the results.  This may help you visualize how you can access more RAM and ROM on the C64.

The program works by switching to the RAM only bank writing a signature high byte to four addresses, then cycles through the standard list of banks reading the value at those locations.  Of course the first line for bank zero shows the bytes as expected.  But the other banks tell a different story.

A000-BFFF is BASIC ROM.   Appears ROM is active in banks 3 & 7 only, otherwise is RAM.

C000-CFFF is always RAM.

D000-DFFF is usually I/O (banks 5-7) including color RAM at D8000 (notice the high nibble changes in this run).  May also appear as RAM (banks 0 & 4), CHAR ROM (banks 1-3).

E000-FFFF is BASIC ROM (banks 2, 3, 6, 7), otherwise RAM (banks 0, 1, 4, 5).

It is interesting to see the similarities and differences in this chart.

The key code to switching banks includes


        lda $01

        and #$f8

        ora #bank_selection

        sta $01


Only clear the interrupt flag (cli instruction) once you have returned to the normal bank (7) so IRQ vectors and the KERNAL/BASIC ROMs will work as expected.