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(notes) 2023-08-21 - Add section brainstorming a revised memory-map + initial Instruction Pointer

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n loewen 2023-08-21 14:34:20 +01:00
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@ -6,15 +6,41 @@
- [x] graphics: 80 col card template - [x] graphics: 80 col card template
- see below - see below
- printing - printing
- [ ] print: paper tape template - [x] print: paper tape template
- [ ] print: colour coding forms - [x] print: colour coding forms
- [ ] print: 80 col card template - [x] print: 80 col card template
- [ ] see code/design commentary below... - [ ] see code/design commentary below...
- (consider python (microbit) version of simulator) - (consider python (microbit) version of simulator)
- maybe ask for e's input - 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)
## Imported notes from earlier ## Imported notes from earlier
### 2023-08-18 cardiograph loose thoughts ### 2023-08-18 cardiograph loose thoughts