The current release is considered fully functional and complete.
Links:
Vic-20 D64 (1541 image) and source
Commodore 64 D64 (1541 image) and source
The Commodore Vic-20 video chip added multiple features beyond what the original PET Model 2001 offered. Besides color support, it also includes the ability to change to user defined character sets. These can be used to customize the look and feel of a program, and also to provide bitmapped graphics.
Back in the 80s I was proud of my development of a character editor program that utilized a few machine language routines to perform quick operations via keyboard selections. Unfortunately I don't have direct access to that program today (haven't been able to read the tape back into the computer yet). So I decided to rewrite it again.
This time it is purely 6502 assembly code. I started using the VICE monitor to write the initial functionality, and soon afterwards transitioned to ACME source to ease edits and additions.
After mocking up the look of the screen, identifying operations that would be nice to have, and testing out memory layout and configuring the video chip to use character images from RAM, I got to work.
1. Initial screen layout including editor and commands.
2. Navigation of character selection, displaying bit patterns for editing to screen, and displaying hex character value and byte values.
3. Editing the pixels in a single character.
4. Saving font to floppy disk.
5. Continuing to iterate with new features and enhancements
Description of key operations
pressing a key on the keyboard as shown below will perform an operation:
@ - toggle between RAM/ROM character sets
SPACE - toggle pixel on/off
B - navigate back one character
C - copy
F - flip upside down
M - mirror the image left to right, right to left
N - navigate forward to next character
R - rotate 90 degress to the right
S - save to FONT.BIN (WARNING: overwrites any previous saves)
V - paste
X - cut
Y - redo
Z - undo
- minus - navigate 16 characters back
+ plus - navigate 16 characters forward
< > - shift pixels left/right
↑ - shift pixels up (^ carrat on PC)
Shift V - shift pixels down
CLR - erase all pixels
CURSOR keys - move cursor left/right/up/down
HOME - move cursor to upper left position
RVS - inverse the pixels
STOP - exit (usually mapped to ESC on PC)
There are up to 24 levels of undo [Z] (25 levels of redo [Y]) when editing a character. Navigating will clear the undos/redos. And there is a virtual clipboard which you can copy[C]/cut[X]/paste[V] a single character. Note in this program you do not use the Ctrl key for clipboard as you do on a PC.
Notes:
- When launching the program for the first time, the ROM font will be copied to RAM, including moving the start of programs up to $1800 past the RAM characters. Subsequent runs from that address will not recopy from ROM. If you want to restart fresh from ROM, either reset your computer and run again, or POKE 44,16:SYS 6157. You don't have to memorize the entry point, you can also LIST the line of BASIC that calls into the entry point.
- Saving overwrites FONT.BIN. So if you want to save your work permanently, it is recommended to rename your file to something else.
OPEN 15,8,15,"R0:COOL=FONT.BIN":CLOSE 15 - To LOAD your font for editing, be sure to run the program at least once, then load before running to enter back into edit mode. Be sure to include the ,1 otherwise you will wipe the font editor out of memory, then best to restart your Commodore and try again.
LOAD "COOL",8,1 - Pressing COMMODORE+SHIFT while in programmable characters will shift to a second set of characters which we don't have room for in RAM, so you will see bitmapped images of the program and video memory. Press the key combination again to return to the first set of characters. [Disabled in the C64 port]
Update 2024 April 26: more documentation
H - hide UI except pixels and character for a minimalist experience
/ - swap between two RAM character sets (C64 version edits total of 512 characters)
F1 - foreground color
F2 - cursor color
F3 - background color
F4 - character set 1 color
F5 - border color
F6 - character set 2 color
F7 - large pixel set color (editor)
F8 - large pixel cleared color (editor)
1,2,3,4,5,6,7,8,9,0 - select theme (colors and grid mode)
As the C64 has a different memory layout, video memory is at a lower address $0400 and more memory available, the memory map is a bit different. The start of the program is still at $1800, but the programmable characters are now from $0800 to $17FF, a full 4K (512 characters).
Note that only the first character set (256 characters) is visible at this address without moving the character set to another location (planned for future). But the editor can handle this by swapping the character sets in RAM for editing, swapping them back and forth, so one set is always edited from the lower $0800-$0FFF range that is visible. Just be sure to swap back to the first set before saving, otherwise the character sets will be out of order.
To use both character sets in your own programs, it is necessary to manually relocate them or load them at another address, unless you use the swapping technique used by the editor.
It is not presently possible to use raster interrupts to show both sets because the second set is at an address that is mapped to ROM characters instead. Another reason it is planned to move these in the future, for example to $3000-$3FFF but in the middle of BASIC RAM it may be too challenging to be useful to others.
Probably a better solution is to move both VIC-II memory to another bank of 16K, which also requires moving video memory. Say bank 4 with characters at $F000-$FFFF, and video ram at $CC00-$CFFF. Then move the start of BASIC to $0400, resulting in 39933 (39K) bytes available for BASIC, an increase of 1K the size of the video screen memory.
Leaving the editor, you have lost 4K of RAM from BASIC. To reclaim 2K for the not visible second character set, POKE 44, 20: POKE 20*256, 0: NEW but don't load any of your double character sets or you will wipe out the first 2K of your BASIC program. Best to leave 4K lost at start of BASIC in that case, or resize your character sets to a single set.
We're in the weeds here. Back to the fun -- please enjoy the programmable character sets and the graphical opportunities they provide!
Update 2024 April 27: raster interrupts implemented as described above, with two sets on screen, and working area using ROM font so doesn't change. Second set is copied to $3800-$3FFF for display purposes, while the two sets remain at $0800-$17FF.
Update 2024 April 28: themes implemented (10) and grid mode added, new C64 keys updated above. Themes allow quickly changing between color sets and grid options using the numeric keys.
Update 2024 May1: wrote an article on moving video and programmable character memory.
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