# Dev notes - 2023-08-21

## Goals for today

- graphic design
  - [x] graphics: 80 col card template
  - see below
- printing
  - [x] print: paper tape template
  - [x] print: colour coding forms
  - [x] 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*](http://wilsonminesco.com/6502primer/MemMapReqs.html)
  - "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?](https://stackoverflow.com/questions/4004493/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)?](https://superuser.com/questions/988473/why-is-the-first-bios-instruction-located-at-0xfffffff0-top-of-ram) (x86)