cardiograph-computer/2023-09-25.md

# 2023-09-25 — Dev notes

## TODO

- [x] Notes on possible architecture extensions to add support for a 16-bit IP
- [ ] add tape drive to mainframe

- review 8080 style io ports
    - works bc they have 2 operands per instruction, i think

- [ ] add notes on re-arranging extended system docs


## Networking - first bits of research

- [Bus network](https://en.m.wikipedia.org/wiki/Bus_network)
- [Econet](https://en.m.wikipedia.org/wiki/Econet)
- [Telnet](https://en.wikipedia.org/wiki/Telnet)
- [Adding a serial port to my 6502 computer – Mike's Software Blog](https://mike42.me/blog/2021-07-adding-a-serial-port-to-my-6502-computer)

- uxn networking — not helpful to me, i think
    - https://github.com/klardotsh/uxnyap
    - https://compudanzas.net/uxn_tutorial_day_7.html
    - https://hacklab.nilfm.cc/xrxs/about/


## 6502 vectors

https://www.masswerk.at/6502/6502_instruction_set.html

> System Vectors
> 
> $FFFA, $FFFB ... NMI (Non-Maskable Interrupt) vector, 16-bit (LB, HB)  
> $FFFC, $FFFD ... RES (Reset) vector, 16-bit (LB, HB)  
> $FFFE, $FFFF ... IRQ (Interrupt Request) vector, 16-bit (LB, HB)  


## Increasing addressing capacity — brainstorm

Increasing the address space would require extending the IP to 16 bits. It would also mean that you would need to be able to provide a 16 bit address when JMPing.

This could add a fair amount of complexity.

Here are a few options I can think of…

### Option 1: 

- Extend the IP to 16 bits
- Add an additional addressing mode for JMP instructions, which takes the high byte from the accumulator and the low byte from memory
- Advantages:
    - Maybe the simplest change?
- Downsides:
    - Annoying to use
    - Can’t easily do an indirect JMP

### Option 2:

(this is what the 6502 does, basically)

- Extend the IP to 16 bits
- Give JMP instructions two bytes of operand
- Advantages:
    - Simple programming
- Disadvantages
    - Variable length instruction set (probably)

### Option 3:

- Extend the IP to 16 bits
- Give JMPs two bytes of operand
- Give _all_ operations two bytes of operand, making it possible to write “LDD addr, addr”
- no i don’t like this

Aside: a few instructions already have a wasted operand byte. (NOP, HLT.) Maybe a variable length instruction set isn’t so bad…? → maybe this is a question to return to, when i get around to trying to build something in a logic simulator

### Option 4:

- Add a “B” register
- ? Add instruction for “Load AB”
- JMP AB
- Advantages
    - Simple enough programming
    - New register might be nice
    - Doesn’t require any ops with 2-byte operands
- Disadvantages
    - Big change
    - Makes the system less simple
    - Need a bunch of new B reg instructions (at least 4: LDB, STB in 2 modes)

### Option 5:

- Extend IP to 16 bits
- Extend A to 16 bits
- Keep the existing instructions — they just all work on the low byte of A now
- Add new mode for LDA and STA, to r/w two bytes, with the starting addr as the operand
- Advantages
    - Simple?
    - All ops still just 2 bytes
- Disadvantages
    - Requires new modes, which are in short supply (and/or requires making the ISA a little less tidy)
    - Maybe makes arithmetic weird

### Option 6:

- this is the same as the last one but presented a bit differently
- add a B register
- add instructions for loading and storing BA as a pair
- JMP uses BA, in direct mode
- everything else (ie, arithmetic) just works on A
- advantages
    - same as above
    - but arithmetic isn’t weird
- disadvantages
    - weird
    - how do you clear B?
        - i guess LDA would reset B?

### Also...

- [ ] TODO: review chip 8 instruction set
- [ ] try 1802-style opcodes... or is that essentially what I already have?


## Idea: Swap Overflow flag for "IO Error"

.


## Links: 1802

- http://t2k.wdfiles.com/local--files/membership-card/RCA1802-Instruction-Set.pdf
- ** http://bitsavers.trailing-edge.com/components/rca/cosmac/MPM-201A_User_Manual_for_the_CDP1802_COSMAC_Microprocessor_1976.pdf


## Links: 6502 history

- https://www.embeddedrelated.com/showarticle/1453.php
- https://www.team6502.org/sydney-anne-holt.html


## Docs leftovers: alternative table format for instructions

| lo ↓ / hi → | 0 (G0, M0) |
|-------------|------------|
| **0**       | END        |
| **1**       | NOP        |
| **2**       |            |
| **3**       |            |
| **4**       |            |
| **5**       |            |
| **6**       |            |
| **7**       |            |
|             |            |
| **8**       |            |
| **9**       |            |
| **A**       |            |
| **B**       |            |
| **C**       |            |
| **D**       |            |
| **E**       |            |
| **F**       |            |

| lo ↓ / hi → | 5 (G1, M1) | 6 (G1, M2) |
|-------------|------------|------------|
| **0**       | LDA #      | LDA ind    |
| **1**       | STO #      | STO ind    |
| **2**       | ADD #      | ADD ind    |
| **3**       | SUB #      | SUB ind    |
| **4**       | JMP #      | JMP ind    |
| **5**       | JEQ #      | JEQ ind    |
| **6**       | JFL #      | JFL ind    |
| **7**       | FTG #      | FTG ind    |
|             |            |            |
| **8**       | MUL #      | MUL ind    |
| **9**       | DIV #      | DIV ind    |
| **A**       | JLT #      | JLT #      |
| **B**       | JGT #      | JGT #      |
| **C**       | NOT #      | NOT #      |
| **D**       | AND #      | AND #      |
| **E**       | OR  #      | OR  #      |
| **F**       | XOR #      | XOR #      |

| lo ↓ / hi → | 9 (G2, M1) | A (G2, M2) |
|-------------|------------|------------|
| **0**       | DEV #      | DEV ind    |
| **1**       | INP #      | INP ind    |
| **2**       | OUT #      | OUT ind    |
| **3**       | FED        | FED        |
| **4**       |            |            |
| **5**       |            |            |
| **6**       |            |            |
| **7**       |            |            |
|             |            |            |
| **8**       |            |            |
| **9**       |            |            |
| **A**       |            |            |
| **B**       |            |            |
| **C**       |            |            |
| **D**       |            |            |
| **E**       |            |            |
| **F**       |            |            |

| lo ↓ / hi → | F (G3, M3) |
|-------------|------------|
| **0**       |            |
| **1**       |            |
| **2**       |            |
| **3**       |            |
| **4**       |            |
| **5**       |            |
| **6**       |            |
| **7**       |            |
|             |            |
| **8**       | RSL A      |
| **9**       | RSR A      |
| **A**       | ASL A      |
| **B**       | ASR A      |
| **C**       |            |
| **D**       |            |
| **E**       |            |
| **F**       |            |