cardiograph-computer/2023-08-21--dev-notes.md

Dev notes - 2023-08-21

Goals for today

• graphic design

  • graphics: 80 col card template
  • see below

• printing

  • print: paper tape template
  • print: colour coding forms
  • print: 80 col card template

• see code/design commentary below...

• (consider python (microbit) version of simulator)

  • maybe ask for e's input

Memory map re-think

CPU start-up

When starting up, the CPU executes a JMP $FF.

Put differently: it starts executing instructions at the address contained in $FF.

Cardiograph memory map

00-19 - display (5x5)
1A    - pointer to display memory
1B    - keypad: value of latest key pressed
1C    - reserved for future use (bank switching flag)
1D-FE - free

References re: where do CPU instruction pointers start, and how are they set?

• "Memory Map Requirements", in 6502 PRIMER: Building your own 6502 computer
  • "Reset (RST): When the 6502's RST input gets pulled low and then brought back high, the 6502 starts its reset process, and gets the address to start executing program instructions from $FFFC-FFFD. Notice it does not start executing at address $FFFC, but reads it to get the beginning address of the routine where it should start executing. That routine will normally have to be in ROM."
• What address does the x86 begin executing at?
  • "The cs (code selector) register is set to 0xffff and ip (instruction pointer) is set to 0x0000."
• Why is the first BIOS instruction located at 0xFFFFFFF0 ("top" of RAM)? (x86)

Imported notes from earlier

2023-08-18 cardiograph loose thoughts

• use binary encoded punch cards for cardiog progs in machine code
• try making a microbit based emulator
  • (microbit + hex keypad)
    • (machine code monitor like VIP…)
  • (+ tape input??)
• a4 template with full size 80 col card
• snake
• [/] add simulator todo: pass asm line thru to cpu to print when debugging
  • asm: create a second array that stores every line with code (nor blank or comment only lines) + its line number
  • cpu: accept an optional debugging array, print line # and statement
• readme:
  • readme: rename (or split up?) mem map / peripherals section
  • ? readme: put 2 keypad charts side by side (they would fit on my phone)
• [/] see paper notes on mem map

2023-08-19

[/] reconsider ISA order in light of supercat comment here - Why didn't the 6503 have increment/decrement opcodes for A?

• [/] look at use of * or . in assembly
  • What does "jmp *" mean in 6502 assembly?

2023-07-17 - cardiograph - worksheet for hand-assembling code… + other docs

• docs/graphics: machine code quick ref (ops + short explanations + mnems)
• docs/graphics: assembly quick ref (as above, plus assembler-specific syntax like constants)
• docs/graphics: worksheet for hand-assembling