cardiograph-computer/2023-08-31.md

Dev notes — 2023-08-31

• parse args in a safer way
• implement missing features
  • single-stepping
  • various levels of debugging, pretty-printing
• add 'dry run' mode to assembler - print debug info but don't write to file
• implement 'jmp ($FF)' on startup
  • implement ROM

Microprocessor trainer style keypad

New keys

I'm thinking the system needs:

1. on/off switch
2. 'shift' key
3. hex keypad

The 'shift' key allows each of the keypad keys to have two functions:

1 / + 2 3 C / ST
4 5 6 D
7 / DA 8 9 E / G1
A / AD 0 B F / GO

(These are based on the KIM-1 buttons)

• Updating and viewing memory:

  • AD n n — go to address nn
  • DA n n — store data at address nn
  • + — go to next address

• Running programs:

  • GO — run, starting at current address
  • G1 — execute next instruction (single-step)
  • ST — stop running

D could be a reset key, but I'm not sure it's necessary?

Usage: inspecting memory

1. Press AD to switch to address-entry mode
2. Enter the address you want to inspect with n n
3. Press + to move to the next address

Usage: entering code

1. Press AD to switch to address-entry mode
2. Enter the starting address with n n
3. Press DA to switch to data-entry mode
4. Enter the data with n n
5. The system automatically advances to the next address; just keep entering data

Usage: running code

Running continuously:

• Press GO to begin continuous execution
• Press ST to stop execution

Single-stepping:

• (Press ST to stop execution)
• Press G1 to execute one instruction

To consider

• If I understand/remember correctly... When the ST button is pressed on the KIM-1, the values in the registers are copied into ram so that they can be inspected
• That seems like a good idea

Handling

I think this is all handled by the I/O controller (or a new DMA controller?) — not the CPU, at any rate

The DMA controller also provides a special "load mode", which allows loading of memory while the CLEAR and WAIT inputs of the processor are active. This allows a program to be loaded without the need for a ROM-based bootstrap loader. This was used by the COSMAC Elf microcomputer and its successors to load a program from toggle switches or a hexadecimal keypad with no required software and minimal hardware. The user could simply set the switches to the next value, toggle the read, and then move on. There was no need to change the addresses, that was done automatically by the DMA stepping.

— https://en.wikipedia.org/wiki/RCA_1802

Displaying data, microprocessor trainer style

The Cardiograph's signature 5x5 display makes this a challenge. I have two ideas:

1 — Display data in binary, with one nibble per line. You can fit two bytes on screen with a space in between.

○ ● ● ● ● F
○ ● ○ ● ○ A
○ ○ ○ ○ ○
○ ○ ● ○ ● 5
○ ● ○ ● ○ A

2 — Display data using a sort of abacus-esque encoding...

8s, 4s ... 3 2 1

○ ○ ○ ○ ○ 0
○ ○ ○ ○ ● 1
○ ○ ○ ● ○ 2
○ ○ ● ○ ○ 3
○ ● ○ ○ ○ 4
○ ● ○ ○ ● 5
○ ● ○ ● ○ 6
○ ● ● ○ ○ 7
● ○ ○ ○ ○ 8
● ○ ○ ○ ● 9
● ○ ○ ● ○ A
● ○ ● ○ ○ B
● ● ○ ○ ○ C
● ● ○ ○ ● D
● ● ○ ● ○ E
● ● ● ○ ○ F

or

○ ○ ○ ○ ○ 0
○ ○ ○ ○ ● 1
○ ○ ○ ● ● 2
○ ○ ● ● ● 3
○ ● ● ● ● 4
○ ● ○ ○ ● 5 = 4 + 1
○ ● ○ ● ● 6 = 4 + 2
○ ● ● ● ● 7 = 4 + 3
● ○ ○ ○ ○ 8
● ○ ○ ○ ● 9 = 8 + 1
● ○ ○ ● ● A = 8 + 2
● ○ ● ● ● B = 8 + 3
● ● ○ ○ ○ C = 8 + 4
● ● ○ ○ ● D = 8 + 4 + 1
● ● ○ ● ● E = 8 + 4 + 2
● ● ● ● ● F = 8 + 4 + 3

the first is maybe clearer as an encoding; more elegant

but mayber the second is easier to quickly read off of an LED display?

Interrupts

From apple note "cardiograph 2023-09-31"

• interrupts…
  • for extended system? (“cardiograph+”?)
• how does a big iron machine like a pdp, burroughs, system/360 load program?
• what are the buttons for memory access on a microprocessor trainer like a kim-1, cosmac?

Startup

From apple note "cardiograph 2023-09-31"

“Typically, a microprocessor will, after a reset or power-on condition, perform a start-up process that usually takes the form of "begin execution of the code that is found starting at a specific address" or "look for a multibyte code at a specific address and jump to the indicated location to begin execution". A system built using that microprocessor will have the permanent ROM occupying these special locations so that the system always begins operating without operator assistance. For example, Intel x86 processors always start by running the instructions beginning at F000:FFF0,[46][47] while for the MOS 6502 processor, initialization begins by reading a two-byte vector address at $FFFD (MS byte) and $FFFC (LS byte) and jumping to that location to run the bootstrap code.[48]”

→ so i could just have a ROM → or maybe two possible configurations? because sth more Cosmac-like or Big Iron-like would be fun too

Extended system

From apple notes -- "features to add / extended system"

• convenience instructions
  • mul
  • div
  • ? rol ror
• ? return stack
• interrupt
• bank switching
• peripherals
  • keypad with DMA for entering programs
  • ?? printer