Thursday, May 9, 2024

Running SWEET16, Steve Wozniak's "The Dream Machine" from WozMon


The story goes that Steve Wozniak was running out of space in Apple Integer BASIC, and out of necessity, determination, and will power, developed SWEET16 to do 16-bit operations in a more compact fashion.  There is already a great article on SWEET16, the interpretive processing 16-bit supplement to 6502 machine language programs, over at 6502.org.  The article tells the history, instructs how to port, and provides a tutorial on how to use it.  (And see article on Wikipedia, and Steve Wozniak's 1977 article in Byte magazine).

My article will follow the steps to produce a binary to run on my minimal 6502 with MC6850 UART, and run through a few examples to use it together with the Apple-1 Hex Monitor, ported from listings included in the Apple-1 Operation Manual (1976).  The Hex Monitor (or WozMon as people generally refer to it) is the original 256-byte ROM included with the Apple-1 computer.  It only needed to include display memory, edit memory, and run from memory operations to initiate control of that computer.

This article and efforts are inspired by videos produced by Ben Eater and his 6502 breadboard computer.  I wanted to do something similar, and followed instructions he provided to do something similar with my 6502 emulator, but simpler.   

I already had a 6502 emulator.  One version is in C++, is very cross-platform (Windows, Linux, Mac, Arduino, STM32, etc.).  But I had focused on Commodore emulation to match the computers I grew up with learning to program.  While that is working well, it is using Commodore ROMs and some I/O emulation. 

Also inspirational are similar videos produced by Michael Cartwright for his Hopper development environment and 6502 breadboard and PCB computer, inspired by Ben Eater's 6502 breadboard computer.  Michael (sillycowvalley on github) uses the MC6850 UART instead of the MC6821 PIA used in Apple-1, or MOS6522 VIA or MOS6551 UART used by Ben Eater.

I surely didn't want to bit bang serial, and wanted it as simple as possible.  Reviewing resources on the web, including datasheets, it was apparent the MC6850 has an advantage of being rather minimalistic.  It has one data register (read/write), one control register (write), and one status register (read).  While originally intended for use with 6800 series microprocessors, it can be used with others.  Only caveat is that the status is not designed for the 6502 BIT instruction.  But the advantage is that it doesn't have a status bug like the 6551 which intended to replace it.

So, I sat down and added a minimal emulation profile to my emulator including configurable RAM/ROM, and I/O address for the MC6850.  My initial implementation supports polling but not interrupts, which is fine for our purposes today.  It does not include a parallel I/O chip, so no hooking up virtual LEDs.  Wait, what?  Oh yeah, we don't need those (yet?), we exclusively have a terminal console.   But that also means we don't have a way to load or save programs.

Also imagine 60K RAM and 4K ROM here (oops!)

I ported WozMon to the 6502/MC6850 minimum profile so it will serve as our launching off point, similar to how it was utilized on the Apple-1.  Porting required changing the assembler syntax to match my 6502 assembler of choice (ACME), especially note that all the hex values in the file should include # for immediate mode (if extracting from Usenet article instead of Byte article), DFB must stand for define byte, and you do need to include some save/restore register routines.

An advantage of having a minimal system unlike Commodore, is that the 32 consecutive zero-page bytes necessary for SWEET16 are guaranteed to be available because it is a minimal system, addresses are not used by IRQ routines, keyboard handlers, timers, KERNAL I/O, etc.

Steps

1. Clone (or download Zip of) the emulator c-simple-6502-cbm.  Note it is not necessary to gather the Commodore ROMs this time around, because we don't need them.

2. Compile using Visual Studio, or make/gcc (Makefile included), or similar.

3. Look for wozmon.bin in the roms/minimal folder (its ported source is in child src folder).

4. Launch the minimal profile

    c-simple-6502-cbm.exe 1 roms/minimal/wozmon.bin

Now WozMon should be running.  You can review the Apple-1 Operation Manual (1976) for more detailed instructions on how to use WozMon.  But if you're quick, you can follow along with the required inputs here.

5. Paste SWEET16 binary into WozMon.   See copyright at start of article, included here for educational purposes, trusting this is fair use and for your own personal use.  Not for commercial use.

# SWEET16 compiled for EE80 start address
EE70: 20 EA EE 68 85 1E 68 85
EE78: 1F 20 7F EE 4C 79 EE E6
EE80: 1E D0 02 E6 1F A9 EF 48
EE88: A0 00 B1 1E 29 0F 0A AA
EE90: 4A 51 1E F0 0B 86 1D 4A
EE98: 4A 4A A8 B9 C8 EE 48 60
EEA0: E6 1E D0 02 E6 1F BD CB
EEA8: EE 48 A5 1D 4A 60 68 68
EEB0: 20 F6 EE 6C 1E 00 B1 1E
EEB8: 95 01 88 B1 1E 95 00 98
EEC0: 38 65 1E 85 1E 90 02 E6
EEC8: 1F 60 00 F7 02 9B 0B 9C
EED0: 23 AD 14 B0 45 B7 4F BE
EED8: 2D C7 59 D0 83 DB 6C 03
EEE0: 31 E6 6E 91 1C E5 63 E5
EEE8: E5 E5 85 20 86 21 84 22
EEF0: 08 68 85 23 D8 60 A5 23
EEF8: 48 A5 20 A6 21 A4 22 28
EF00: 60 10 B3 B5 00 85 00 B5
EF08: 01 85 01 60 A5 00 95 00
EF10: A5 01 95 01 60 A5 00 81
EF18: 00 A0 00 84 1D F6 00 D0
EF20: 02 F6 01 60 A1 00 85 00
EF28: A0 00 84 01 F0 ED A0 00
EF30: F0 06 20 64 EF A1 00 A8
EF38: 20 64 EF A1 00 85 00 84
EF40: 01 A0 00 84 1D 60 20 24
EF48: EF A1 00 85 01 4C 1D EF
EF50: 20 15 EF A5 01 81 00 4C
EF58: 1D EF 20 64 EF A5 00 81
EF60: 00 4C 41 EF B5 00 D0 02
EF68: D6 01 D6 00 60 A0 00 38
EF70: A5 00 F5 00 99 00 00 A5
EF78: 01 F5 01 99 01 00 98 69
EF80: 00 85 1D 60 A5 00 75 00
EF88: 85 00 A5 01 75 01 A0 00
EF90: F0 E9 A5 1E 20 17 EF A5
EF98: 1F 20 17 EF 18 B0 0E B1
EFA0: 1E 10 01 88 65 1E 85 1E
EFA8: 98 65 1F 85 1F 60 B0 EC
EFB0: 60 0A AA B5 01 10 E8 60
EFB8: 0A AA B5 01 30 E1 60 0A
EFC0: AA B5 00 15 01 F0 D8 60
EFC8: 0A AA B5 00 15 01 D0 CF
EFD0: 60 0A AA B5 00 35 01 49
EFD8: FF F0 C4 60 0A AA B5 00
EFE0: 35 01 49 FF D0 B9 60 A2
EFE8: 18 20 64 EF A1 00 85 1F
EFF0: 20 64 EF A1 00 85 1E 60
EFF8: 4C AE EE 00 00 00 00 00

Note: SWEET16 entry point for this port is $EE70, and the operations are in a single page $EF01-$EFFA.   A save registers routine is at $EEEA and restore registers is at $EEF6.   Zero page usage is $00-$1F for the SWEET16 registers, and $20-$23 for saving 8-bit registers.

6. Let's try it out.  Start by pasting the following to enter a program at 

1000: 20 00 FE A9 2A 20 70 EE 
1008: 11 12 34 12 56 78 21 A2
1010: 33 00 8D F8 FF 00

Which is equivalent to

20 00 FE JSR $FE00
A9 2A    LDA #$2A
20 70 EE JSR $EE70
11 12 34 SET R1,$3412
12 56 78 SET R2,$7856
21       LD  R1 
A2       ADD R2
33       ST  R3
00       RTN
8D F8 FF STA $FFF8
00       BRK

JSR $EE70 switches to SWEET16 interpretation, and these are SWEET16 instructions through RTN, before being back in 6502.  The vectors are set such that a BRK will return to WozMon. ($FE00 is my specific UART initialization routine for MC6850, and storing a byte to $FFF8 will send it across the UART).

All this program does is set the R1 and R2 16-bit registers to constant values, the LD instruction loads the 16-bit accumulator from R1, then ADD R2 adds R2 to the accumulator, and the result is stored in R3 by the ST instruction.   RTN returns to normal 6502 operation.

7. To run the program, enter the following into WozMon:

1000R

8. Display the results by entering

0.1FR

From the last two steps the output should look something like the following (blank lines added for separation)

1000R

1000: 20*\


0.1F

0000: 68 AC 12 34 56 78 68 AC
0008: 00 00 00 00 00 00 00 00
0010: 00 00 00 00 00 00 00 00
0018: 00 00 00 00 00 06 12 10

And the accumulator (R0) at addresses 0 and 1 now has the value $AC68 which appears to be the sum of the other two 16-bit values, with a copy also stored at R3 (addresses 6/7).  The asterisk is printed to demonstrate registers being saved/restored and 6502 operation returning to normal.

That's just a quick example.   Hopefully you've got a little taste of WozMon and SWEET16. 

Example run of SWEET16

But that's not all!  

I made a Commodore port of WozMon, so why not also port SWEET16 to Commodore and run it there too?

# Commodore port of SWEET16
1540: 85 22 86 23 84 24 08 68
1548: 85 25 d8 60 a5 25 48 a5
1550: 22 a6 23 a4 24 28 60 00
1558: 00 00 00 00 00 00 00 00
1560: 00 00 00 00 00 00 00 00
1568: 00 00 00 00 00 00 00 00
1570: 00 00 00 00 00 00 00 00
1578: 00 00 00 00 00 00 00 00
1580: 20 40 15 68 85 7e 68 85
1588: 7f 20 8f 15 4c 89 15 e6
1590: 7e d0 02 e6 7f a9 16 48
1598: a0 00 b1 7e 29 0f 0a aa
15a0: 4a 51 7e f0 0b 86 7d 4a
15a8: 4a 4a a8 b9 d8 15 48 60
15b0: e6 7e d0 02 e6 7f bd db
15b8: 15 48 a5 7d 4a 60 68 68
15c0: 20 4c 15 6c 7e 00 b1 7e
15c8: 95 61 88 b1 7e 95 60 98
15d0: 38 65 7e 85 7e 90 02 e6
15d8: 7f 60 00 f7 02 9b 0b 9c
15e0: 23 ad 14 b0 45 b7 4f be
15e8: 2d c7 59 d0 83 db 6c 03
15f0: 31 e6 6e 91 1c e5 63 e5
15f8: e5 e5 00 00 00 00 00 00
1600: 00 10 c3 b5 60 85 60 b5
1608: 61 85 61 60 a5 60 95 60
1610: a5 61 95 61 60 a5 60 81
1618: 60 a0 00 84 7d f6 60 d0
1620: 02 f6 61 60 a1 60 85 60
1628: a0 00 84 61 f0 ed a0 00
1630: f0 06 20 64 16 a1 60 a8
1638: 20 64 16 a1 60 85 60 84
1640: 61 a0 00 84 7d 60 20 24
1648: 16 a1 60 85 61 4c 1d 16
1650: 20 15 16 a5 61 81 60 4c
1658: 1d 16 20 64 16 a5 60 81
1660: 60 4c 41 16 b5 60 d0 02
1668: d6 61 d6 60 60 a0 00 38
1670: a5 60 f5 60 99 60 00 a5
1678: 61 f5 61 99 61 00 98 69
1680: 00 85 7d 60 a5 60 75 60
1688: 85 60 a5 61 75 61 a0 00
1690: f0 e9 a5 7e 20 17 16 a5
1698: 7f 20 17 16 18 b0 0e b1
16a0: 7e 10 01 88 65 7e 85 7e
16a8: 98 65 7f 85 7f 60 b0 ec
16b0: 60 0a aa b5 61 10 e8 60
16b8: 0a aa b5 61 30 e1 60 0a
16c0: aa b5 60 15 61 f0 d8 60
16c8: 0a aa b5 60 15 61 d0 cf
16d0: 60 0a aa b5 60 35 61 49
16d8: ff f0 c4 60 0a aa b5 60
16e0: 35 61 49 ff d0 b9 60 a2
16e8: 18 20 64 16 a1 60 85 7f
16f0: 20 64 16 a1 60 85 7e 60
16f8: 4c be 15

This SWEET16 entry point for Commodore changed to $1580 so the sample program has to change to call this entry point instead, to call $FFD2 for screen output instead of using the UART, and jump to WozMon entry at end instead of BRK.   Note that this Commodore port puts the SWEET16 registers at $60 (instead of $00) in zero page.   The Commodore port was trivial, disassembly will show there's nothing secret going on, mostly moving around stuff in zero page, and moving the save/load registers code.

1000: A9 2A 20 80 15 11 12 34
1008: 12 56 78 21 A2 33 00 20
1010: D2 FF 4C 00 14
 
Vic-20 running WozMon and SWEET16

C64 running WozMon and SWEET16

It appears there's a bug restoring the registers.  That is not an asterisk!!!   But an underscore.   Exercise left for another day...   Update: I had replaced the call to UART_INIT from the other system with starting SWEET16 followed by 6502 code.  Bad, so bad (see screenshot for bad code).  Have since updated the sample program to be correct.   Disassembling here to double check.   The asterisk is now showing up fine!  Would have been doing some crazy stuff interpreting 6502 opcodes as SWEET16.

A9 2A    LDA #$2A
20 80 15 JSR $1580
11 12 34 SET R1,$3412
12 56 78 SET R2,$7856
21       LD  R1 
A2       ADD R2
33       ST  R3
00       RTN
20 D2 FF JSR $FFD2
4C 00 14 JMP $1400


Wednesday, May 1, 2024

C64 RAM locations for programmable characters

 


Taking the information from Jim Butterfield's interactive program we can determine a few optimal choices for configuring the location of video and character set memory.  They both need to be in the same 16K bank of the C64 memory, video memory needs to be accessible to the KERNAL, and some locations are off limits (offsets $1000-$1FFF within the bank) because the VIC-II only sees D000-DFFF ROM characters there.

Option 1: easiest, video $0400, chars $2000-$2FFF

Option 2: least BASIC free, video $8C00, chars $A000-$AFFF

Option 3: largest free, video $CC00, chars $F000-$FFFF


Option 4: single charset, video $C400, chars $C800-$CFFF


Thanks to Jim Butterfield's SCREENMAP 64 program, it prompts for you to choose between valid options, and then calculates the register and screen page values that you need to change(poke).  Shown above are details for the four options that seem best.  The first three allow for two full sets of characters.  The last one allows for a single character set, or one and a half character sets.

In addition to the steps shown, it may be necessary to install an NMI handler to keep the values set instead of reset to ROM defaults, and it will be necessary to load a font into the desired programable character set address range.

Options 3 and 4 are interesting because they both increase BASIC RAM by 1K more than normal by moving video screen memory elsewhere. Option 1 is the easiest to implement, so is perfect for getting started.


Notes:

1000-1FFF not available for VIC because maps to D000 ROM
5000-5FFF not available for VIC because maps to D000 ROM
9000-9FFF not available for VIC because maps to D000 ROM
D000-DFFF not available for VIC because maps to D000 ROM

C000-CFFF not recommended for prog. chars. because other RAM in that bank not usually available to KERNAL needed for video screen characters.   So if screen characters are there, only room for one set (2K) instead of two sets (4K)

Option 0 (Normal)
0400-07FF screen memory (1K) = 1024
0800-9FFF BASIC RAM (38K) 
C000-CFFF free (4K)
D000-DFFF character ROM banked under I/O

Option 1 (First 16K)
0400-07FF screen memory (1K) = 1024
0800-1FFF free (6K)
2000-2FFF prog. chars. (4K) = 8192
3000-9FFF BASIC RAM (28K)

4000-7FFF is smack dab in the middle of BASIC RAM, seems like a bad idea unless used with 6502 machine language or other assets around it, and not strictly BASIC.

Option 2 (Third 16K)
0400-8BFF BASIC RAM (34K)
8C00-8FFF screen memory (1K) = 35840
9000-9FFF free (4K)
A000-AFFF prog. chars in RAM bank under BASIC ROM = 40960

Option 3 (Fourth 16K)
0400-9FFF BASIC RAM (39K)
C000-CBFF free (3K)
CC00-CFFF screen memory (1K) = 52224
E000-EFFF = 57344 prog. chars in RAM bank under KERNAL ROM *or* F000-FFFF = 61440

Option 4 (Fourth 16K)
0400-9FFF BASIC RAM (39K)
C000-C3FF free (1K) or half lowercase character set (no inverse characters)
C400-C7FF screen memory (1K) = 50176
C800-CFFF prog. chars (1 set, 2K only) = 51200


Tuesday, April 30, 2024

Raster Interrupts for C64

 


The VIC II chip (video IC) that gives the C64 a lot of its characters (pun intended), includes registers to set up interrupts at certain raster lines.  This allows for video tricks.  Above the background color is changed in the middle of the screen, and changed back when in the borders before drawing the screen again.


This code was leveraged from my character editor source for c64.  That project needed to display multiple different character sets (from ROM and RAM) on the screen simultaneously.   And that is described at the bottom of an earlier blog post.

My approach was to launch Vice x64sc (Commodore 64 emulator) and utilize its built in monitor (ALT-H) to craft prototype code.  The listings following are what I came up with.   Then I used the clipboard to move to a source editor, added labels, and further refined to change from background colors to font changes.

Written by Dave VW 2024-04-27

*** NOTE THIS IS THE ROUGH DRAFT
*** SOURCE CODE AT GITHUB IS MUCH CLEANER
*** ASSEMBLY SOURCE INSTEAD OF MONITOR LISTING

This part was created in the monitor...

>C:c000  2c 19 d0 10  44 ea ad 19  d0 29 01 f0  3b 2c 00 c1
>C:c010  30 16 10 60  ea a9 00 8d  12 d0 ad 11  d0 29 7f 8d
>C:c020  11 d0 0e 00  c1 4c 3b c0  ce 21 d0 a9  a4 8d 12 d0
>C:c030  ad 11 d0 29  7f 8d 11 d0  4e 00 c1 a9  01 8d 19 d0
>C:c040  ea 68 a8 68  aa 68 40 00  ea 4c 31 ea  a9 40 8d 00
>C:c050  c1 78 a9 80  8d 12 d0 ad  11 d0 29 7f  8d 11 d0 a9
>C:c060  00 8d 14 03  a9 c0 8d 15  03 ad 1a d0  09 01 8d 1a
>C:c070  d0 58 60 00  ad 12 d0 c9  c2 90 f9 a2  0a ca d0 fd
>C:c080  ee 21 d0 4c  15 c0                                

.C:c000  2C 19 D0    BIT $D019
.C:c003  10 44       BPL $C049
.C:c005  EA          NOP
.C:c006  AD 19 D0    LDA $D019
.C:c009  29 01       AND #$01
.C:c00b  F0 3B       BEQ $C048
.C:c00d  2C 00 C1    BIT $C100
.C:c010  30 16       BMI $C028
.C:c012  10 60       BPL $C074
.C:c014  EA          NOP
.C:c015  A9 00       LDA #$00
.C:c017  8D 12 D0    STA $D012
.C:c01a  AD 11 D0    LDA $D011
.C:c01d  29 7F       AND #$7F
.C:c01f  8D 11 D0    STA $D011
.C:c022  0E 00 C1    ASL $C100
.C:c025  4C 3B C0    JMP $C03B
.C:c028  CE 21 D0    DEC $D021
.C:c02b  A9 A4       LDA #$A4
.C:c02d  8D 12 D0    STA $D012
.C:c030  AD 11 D0    LDA $D011
.C:c033  29 7F       AND #$7F
.C:c035  8D 11 D0    STA $D011
.C:c038  4E 00 C1    LSR $C100
.C:c03b  A9 01       LDA #$01
.C:c03d  8D 19 D0    STA $D019
.C:c040  EA          NOP
.C:c041  68          PLA
.C:c042  A8          TAY
.C:c043  68          PLA
.C:c044  AA          TAX
.C:c045  68          PLA
.C:c046  40          RTI
.C:c047  00          BRK
.C:c048  EA          NOP
.C:c049  4C 31 EA    JMP $EA31
.C:c04c  A9 40       LDA #$40
.C:c04e  8D 00 C1    STA $C100
.C:c051  78          SEI
.C:c052  A9 80       LDA #$80
.C:c054  8D 12 D0    STA $D012
.C:c057  AD 11 D0    LDA $D011
.C:c05a  29 7F       AND #$7F
.C:c05c  8D 11 D0    STA $D011
.C:c05f  A9 00       LDA #$00
.C:c061  8D 14 03    STA $0314
.C:c064  A9 C0       LDA #$C0
.C:c066  8D 15 03    STA $0315
.C:c069  AD 1A D0    LDA $D01A
.C:c06c  09 01       ORA #$01
.C:c06e  8D 1A D0    STA $D01A
.C:c071  58          CLI
.C:c072  60          RTS
.C:c073  00          BRK
.C:c074  AD 12 D0    LDA $D012
.C:c077  C9 C2       CMP #$C2
.C:c079  90 F9       BCC $C074
.C:c07b  A2 0A       LDX #$0A
.C:c07d  CA          DEX
.C:c07e  D0 FD       BNE $C07D
.C:c080  EE 21 D0    INC $D021
.C:c083  4C 15 C0    JMP $C015

The initialization routine is at $c04c to install the new IRQ handler $c000.  The code jumps around a bit and has NOP (no instruction) as a by-product of patching the source in the monitor to get the desired functionality.   This is cleaned up a lot more in the sources to both repos.

Monday, April 29, 2024

MLX II Listing for C64 Character Sets Editor


Shown with 80s theme colors (key: 6)

Back in the day we would enter program listings from magazines.  That's how we got new utility and game software on a monthly basis.  It was a fun activity in front of my Commodore Vic-20 on Saturdays.

For nostalgia, and retro vibes, you can repeat the process to get this character editor program.   It's online elsewhere and pointed to in this blog, but if you have an air gapped C64, you can enter directly.

Note: Floppy drive 8 is required for loading/saving fonts and theme configuration.  This program does not support tape drives or alternate floppy drives.  So be prepared with a floppy drive 8 such as 1541, 1571, 1581.

Please enter the hex listing below using Compute's Gazette MLX II which includes a ninth checksum byte at the end of each line.  MLX will stop you from continuing if the checksum doesn't match it's internal algorithm.

e.g. I test entered using 64 MLX from Compute's Gazette disk from November 1993 

start:  0801
finish: 24b8

0801:0b 18 0a 00 9e 36 31 35 43
0809:37 00 00 00 00 00 00 00 b4

(gap of zeros or unused, use STOP+RESTORE if get stuck)

1800:00 0b 18 0a 00 9e 36 31 ae
1808:35 37 00 00 00 20 17 1f 6e
1810:18 66 a6 18 6e b1 24 a9 68
1818:00 85 22 a9 08 85 23 20 45
1820:a3 20 ad 18 d0 29 f0 8d fb
1828:b3 24 09 04 8d b4 24 20 44
1830:97 21 20 2b 20 a5 2c c9 e4
1838:18 f0 36 a9 01 a2 18 85 5a
1840:2b 86 2c a9 00 85 fb 85 5b
1848:fd a9 08 85 fc a2 d0 86 d5
1850:fe a9 10 85 ff a0 00 78 bf
1858:20 63 1f b1 fd 91 fb c8 67
1860:d0 f9 e6 fc e6 fe c6 ff e4
1868:d0 f1 20 5c 1f 58 20 7c 5e
1870:1f a9 93 20 d2 ff a9 13 0c
1878:20 d2 ff 2c b1 24 30 44 f2
1880:a9 a1 a2 22 20 8e 1d a9 83
1888:30 a2 23 85 b4 86 b5 20 81
1890:9f 1d 20 cc 1e a9 30 85 26
1898:fd a9 08 85 ff a9 be a2 52
18a0:23 20 8e 1d a5 fd 20 d2 46
18a8:ff a9 b3 a2 23 20 8e 1d b7
18b0:a9 20 20 d2 ff 20 9f 1d cb
18b8:e6 fd c6 ff d0 df 20 db d6
18c0:20 20 9f 1d 20 e1 1e a9 3d
18c8:00 85 27 a9 00 85 28 a9 e9
18d0:38 a2 04 85 29 86 2a a2 f8
18d8:d8 85 b2 86 b3 18 a5 a2 81
18e0:69 04 85 a3 20 b9 1d a0 74
18e8:00 b1 29 85 a7 ad 18 d0 f7
18f0:29 fe c9 10 d0 05 a9 12 af
18f8:8d 18 d0 20 8b 1e 20 e4 0c
1900:ff f0 ea 48 20 bb 1e 68 e4
1908:c9 4e d0 19 20 17 1f a5 9f
1910:22 69 07 85 22 90 c6 e6 ae
1918:23 a5 23 c9 10 90 be a9 30
1920:08 85 23 d0 b8 c9 2b d0 3d
1928:19 20 17 1f a5 22 69 7f cb
1930:85 22 90 a9 e6 23 a5 23 8c
1938:c9 10 90 a1 a9 08 85 23 1b
1940:d0 9b c9 42 d0 1a 20 17 65
1948:1f a5 22 e9 08 85 22 b0 a1
1950:8c c6 23 a5 23 c9 08 b0 3a
1958:84 a9 0f 85 23 4c dd 18 8f
1960:c9 2d d0 1a 20 17 1f a5 bf
1968:22 e9 80 85 22 b0 0c c6 41
1970:23 a5 23 c9 08 b0 04 a9 53
1978:0f 85 23 4c dd 18 c9 11 b0
1980:d0 31 a5 29 69 27 85 29 ca
1988:85 b2 90 05 e6 2a e6 b3 ed
1990:18 a5 28 69 01 29 07 85 14
1998:28 d0 15 38 a5 29 e9 40 1f
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Friday, April 19, 2024

Edit Programmable Characters on Vic-20 and C64

 



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 20: also ported to Commodore 64

Update 2024 April 26: more documentation

Description of key operations since added to Commodore 64 version

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)

, - change titles color

. - change menus color

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 May 1: wrote an article on moving video and programmable character memory.

Update 2024 May 4: have fixed many issues, and more enhancements, including moving programmable characters to $2000-$2FFF to make more usable, eliminate need to have separate edit/view copies.  But loses more BASIC RAM ($0800-$1FFF 6K now available for other uses) while reserving memory for characters.

Primary Color Theme