![]() ![]() ![]() I don’t want a full assembler or typewrite keyboard, but I do have a lot of spare shift-keys, so it might be fun to put frequently used opcodes as shortcuts on some spare keys, Sinclair ZX81/Spectrum-style. I could make it easier to write text across two lines, for example. There’s almost certainly a better way of doing this. At the moment you have to load the ASCII value of the character you want to show into the accumulator and call a subroutine. I could also make improvements to the ROM to make displaying text easier. Straight away, for example, I saw he’d used logical shifts instead of rotates – if I’d done that I’d never have had the carry bug that was corrupting my display of hexadecimal numbers. The Apple 1 computer had an amazingly compact monitor program written by Steve Wozniak, usually called ‘wozmon’ – the source code is available and it’ll be instructional to dig into it and see how it works. There are stacks of books and websites to mine, from classics like Programming the 6502 by Rodnay Zaks, to helpful blogs like this one and this interactive one. There’s a lot I can do with this computer to learn more about 6502 machine code. So it’s time to take stock and consider what next… It has a hexadecimal keypad for entering instructions, a 16×2 alphanumeric LCD display and a very basic monitor program that allows me to enter programs up to 256 bytes in length and run them. ![]() I recently achieved my goal of making a 1970s-style 6502 breadboard computer that I can use to practice machine code programming. ![]()
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