## [DRAFT] Design of the _Cardiograph Mark I_ mainframe computer

The components of a Mark I are:

- a CG 101 Central Processing Unit
- a CG 102 Core Memory Unit
- a CG 103 Print-Key-Punch
- a CG 104 Matrix Display

### Operator console

<mark>TBC TBC TBC</mark>

## Basic controls

- Power
- Load
- Run
- Halt
- 4 Sense Switches

## Status lights

- 8 Accumulator lights
- 8 Address lights
- 8 Data lights
- 8 Instruction Pointer lights (<mark>review IP size?</mark>)
- 8 Status Register lights

## Debugging controls

- Run Single Step
- Memory Read
- Memory Read Next
- Memory Write
- Memory Write Next

## IO programming

Only one input or output device can be accessed at a time.

### Reading data

1. Use `DEV xx` to select input device _xx_
2. Use `INP yy` to read one byte into memory at address _yy_

<mark>TODO: find a way to allow the input device to refuse to provide input</mark>

### Writing data

1. Use `DEV xx` to select output device _xx_
2. Use `OUT yy` to write one byte from memory at address _yy_
3. Use `FED xx` to...
  - card punch: load a new card
  - printer: begin a new line (CR, LF)
  - display: begin a new line

### Punched card format

FIXME:
- ~~Cards are punched in EBCDIC~~
- ~~EBCDIC data is translated into binary by the card reader/punch~~
- Only columns 1-64 are used (for a maximum of 64 bytes of data per card)

### Printer format

- The printer format is the same as the card format
- One line of printing is equivalent to one card
- The printer can print up to 64 characters per line

### Matrix display format

- The display is a 5x5 grid of lights
- Each light has 16 possible brightness levels (0 = off, 15 = maximum)
- The display is written one line at a time
- After the display is selected with `DEV`, writing begins on the top line
- Writing wraps around and begins at the top again, if more than 5 lines are written

### Device numbers

1. card reader / typewriter
2. card punch / line printer
3. display

### Print-Key-Punch configurations

A dial allows you to select which input device to connect to the CPU:

1. none
2. card reader
3. keyboard

A similar dial selects the output device to connect:

1. none
2. card punch
3. printer

Thus, this all-in-one device allows the following configurations:

|                 | printer              | card punch       | none                     |
|-----------------|----------------------|------------------|--------------------------|
| **keyboard**    | ***teletypewriter*** | ***auto punch*** | ***keypunch (offline)*** |
| **card reader** | (keys + print)       | card duplicator  | (card reader)            |
| **none**        | line printer         | (auto punch)     | (scrap metal)            |