cardiograph-computer/2023-08-21.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)