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Sunday, 24 May 2015

Commodore 4032 Editor ROM for Business keyboard

After fixing the RAM fault on a 4032 recently, there was another request to deal with.
The owner plans to install the board in an 8032 case. It should fit fine as boards are identical barring a few missing chips on the 4032, as are the cases, power supplies and monitors. Other than the screen size, the other important difference between the later 4032 and 8032 models is the keyboard.
There are two of mine. The one on the left is a slightly older model with a taller 'shoulder', but the 4032 and 8032 appear with both heights.
This is the 4032, it has the 40 column screen and the 'normal' or 'graphics' keyboard, a direct ancestor of the chiclet keyboards on the original 2001 series pets.
It is unusual in that the top row has only symbols on it. No numbers. There is a 16 key numeric keypad on the right, and these are the only source of numbers. It can be tricky to use if you are switching between using this and any other keyboard on the planet.
This is an 8032, it has an 80 column screen and the 'business' keyboard. This has an 11 key numeric keypad on the left, and the the top row is a the standard arrangement of numbers with symbols above.
This keyboard is the only one available on later 8032-SK models, and the basis of the ones used on CBM-II etc. The 4032 was the last one to have the graphics keyboard.
Early VIC20's actually had what look like pet business keyboards with the function keys using the middle row of the keypad keys and the other two rows taped up.
These keyboards are not interchangeable though, so the key matrix on the PCB must have been changed. The black keyboard at the top is the CBM business keyboard (from an 8032-SK). The bottom is an early VIC20. The square font and the key shape is the same. Some keys have been moved around, TAB becomes CTRL etc. I need to refurbish this VIC20 keyboard, so I'll try to do a pet business keyboard at the same time for comparison.
So back to the problem. The repaired 4032 board is expecting to be connected to a graphics keyboard, and the 8032 case the owner has is fitted with a business keyboard. The keymap is one of the things set by the editor ROM in the pet, other things being the screen size and refresh rate. So it is just a case of locating the appropriate editor ROM image. All of the known versions are available on the Commodore archive. The various combinations of keyboard and screen are there, these are the ones available for BASIC 4 machines with a CRTC (the later 4032 and 8032 machines).

  • 901498-01 - edit-4-40-n-50Hz (40 column, normal keyboard UK)
  • 901499-01 - edit-4-40-n-60Hz (40 column, normal keyboard US)
  • 901474-04 - edit-4-80-b-50Hz (80 column, business keyboard UK)
  • 901474-03 - edit-4-80-b-60Hz (80 column, business keyboard US)

These are the 4 versions Commodore originally produced, the 9014xx numbers are the Commodore part numbers, so the standard UK 8032 has the 901474-04 chip in the UD7 socket.
The one that would be required for the 4032 with a business keyboard would be edit-4-40-b-60Hz (he is in America, so needs the 60Hz version). Commodore didn't make a machine with this combination, so no ROM image appears to exist. Steve Gray has a project to generate new editor ROMs, that adds all sort of nice features, but unfortunately, the 40 column version is currently incomplete. As part of this project, he has disassembled and annotated a number of editor ROMs, including some ones with N and B keyboards. He kindly made those available and I was able to compare the relevant sections. The main change is a 64 byte block at the end which is the 8x8 key matrix lookup table.
Copying the matrix from an 8032 ROM into the 4032 ROM gave me an almost working keyboard. The numbers and some of the symbol keys were incorrect. Comparing the code further, I found a section which deals with the shift key. Here I've disassembled the relevant sections of the 901498-01 4032 ROM and the 901474-04 8032 ROM using CBMxfer viewer. There any many differences between the two ROMs, mainly to do with the screed editing.
The sections in blue are the same. The code on the left is the 4032 ROM disassambly which as two sections of NOPs. The code on the right is from the 8032 ROM, and has some code in there which is checking for certain key ranges. If it is a number key (ASCII / PETSCII 0x31-0x39), and shift is held, it subtracts 0x10, giving codes 0x21-0x29 (symbols !"#$%^'()). Some other keys get 0x80 added (right arrow = 0x1D, + shift = left arrow, 0x9D etc.)  The section of code is in a slightly different location, but the jumps are all relative, and the replacement code turns out to be exactly the same size as the NOPs, so I overwrote the NOP instructions with the code copied from the 8032 ROM and it is now working perfectly. I presume there was some common ancestor to both these ROMS with a business keyboard.
I've burned the image onto a 28C16, and tested the board with a business keyboard and all seems to be working well. There are a few programs which seem to bypass the editor ROM and access the keyboard directly, and example being space invaders. This doesn't work on an 8032 either, so it is expecting a graphics keyboard and scanning it directly.
Whilst I had everything to hand, I also generated an edit-4-40-b-50Hz version. I have submitted these to the archive in case anyone needs this combination again in the future.