242 lines
8.0 KiB
Markdown
242 lines
8.0 KiB
Markdown
# Cardiograph computers
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The Cardiographs are a pair of imaginary computers.
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The "Cardiograph Mark I" is an educational model of a mainframe machine.
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The "MicroCardiograph" is a its miniaturized descendent, a microprocessor trainer.
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They use the same instruction set and have very similar CPUs.
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The main difference is in their peripheral hardware:
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the Mark I is designed for batch processing programs on punched cards,
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while the MicroCardiograph is designed to be used interactively.
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The Cardiographs were built by an imaginary enterprise, the Electronic Computer Group (ECG).
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## Simulator
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There is a [simulator](micro/readme-micro.md) for the MicroCardiograph.
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## CPU
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### Registers
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There are three 8-bit registers:
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1. **A**, the accumulator (and the only general-purpose register)
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2. **IP**, the instruction pointer (aka program counter)
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3. **Status**
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#### Status register
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The *high byte* holds the ID number of the current **IO** device. (See the section on [IO programming](#io-programming).)
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The *low byte* holds four flags:
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**O**verflow, **N**egative, **Z**ero, and **C**arry.
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The flags are accessed by number:
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| O | N | Z | C |
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|---|---|---|---|
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| 8 | 4 | 2 | 1 |
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### Instruction set
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- Instructions are two bytes long:
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one byte for the opcode, one for the operand
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```GGMM IIII``` - **G**roup, **M**ode, **I**nstruction
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| lo ↓ / hi → | 0 (G0, M0) | 5 (G1, M1) | 6 (G1, M2) | 9 (G2, M1) | A (G2, M2) |
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|-------------|------------|------------|------------|------------|------------|
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| **0** | END | LDA # | LDA ind | DEV # | DEV ind |
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| **1** | NOP | STO # | STO ind | INP # | INP ind |
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| **2** | | ADD # | ADD ind | OUT # | OUT ind |
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| **3** | | SUB # | SUB ind | FED | FED |
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| **4** | | JMP # | JMP ind | | |
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| **5** | | JEQ # | JEQ ind | | |
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| **6** | | JFL # | JFL ind | | |
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| **7** | | FTG # | FTG ind | | |
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| | | | | | |
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| **8** | | MUL # | MUL ind | | |
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| **9** | | DIV # | DIV ind | | |
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| **A** | | RRL # | RRL ind | | |
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| **B** | | RRR # | RRR ind | | |
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| **C** | | ARL # | ARL ind | | |
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| **D** | | ARR # | ARR ind | | |
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| **E** | | JLT # | JLT ind | | |
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| **F** | | JGT # | JGT ind | | |
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- RRL/RRR: Ring Rotate
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- JLT: Jump Less Than
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- DEV: IO device select
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- FED: "feed" - line feed / end of card
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<mark>TODO: assess JMPs vs. HOPs</mark>
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### Connections (pinout)
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<mark>TBC</mark>
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| number | name | in/out? | description |
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|---------|-----------|---------|---------------|
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| 1 | RST | in | *reset* |
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| 2 | VCC | in | *power* |
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| 3 | GND | in | *ground* |
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| 4 | CLK | in | *clock* |
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| 5 - 13 | A0 - A7 | out | *address bus* |
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| 15 - 23 | D0 - D7 | out | *data bus* |
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| 24 | ABE | out | *address bus enable*: <br> low when the CPU is using the address bus |
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| 25 | DBE | out | *data bus enable*: <br> low when the CPU is using the data bus |
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| 26 | WAIT | in | *wait* — when pulled low, <br> the current operation is completed <br> and then execution pauses |
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### Start-up
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<mark>TODO: see if this makes sense for the mainframe </mark>
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When starting up, the CPU executes a `JMP $FF`.
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Put differently: it starts executing instructions at the address contained in `$FF`.
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<mark>TODO: currently the simulator doesn't actually do this</mark>
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## Cardiograph Mark I (mainframe)
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The components of a Mark I are:
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- an ECG 101 Central Processing Unit
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- an ECG 102 Core Memory Unit
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- an ECG 103 Card Reader
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- an ECG 104 Card Punch
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- an ECG 105 Line Printer
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- an ECG 106 Matrix Display
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Additionally, an *ECG 100 Keypunch* is used for the initial preparation of cards or tape.
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### Console
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The console is equipped with:
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- Power switch
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- Load button
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- Run button
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- Run Single Step button
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- Halt button
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- Memory Read button
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- Memory Read Next button
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- Memory Write button
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- Memory Write Next button
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- 16 Sense switches (<mark>TBC</mark>)
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- 8 Accumulator lights
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- 8 Address lights
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- 8 Data lights
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- 8 Instruction Pointer lights (<mark>TBC</mark>)
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- 4 Status Register lights
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## IO programming
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Only one input or output device can be accessed at a time.
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### Reading data
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1. Use `DEV xx` to select input device _xx_
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2. Use `INP yy` to read one card into memory, beginning at address _yy_
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### Writing data
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1. Use `DEV xx` to select output device _xx_
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2. Use `OUT yy` to write one byte
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3. Use `FED xx` to signal the end of a card, or the end of a line on the printer or display
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### Punched card format
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- Cards are punched in EBCDIC
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- EBCDIC data is translated into binary by the card reader/punch
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- Only columns 1-64 are used (for a maximum of 64 bytes of data per card)
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### Printer format
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The printer can print up to 64 characters per line.
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### Matrix display format
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- The display is a 5x5 grid of lights
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- Each light has 16 possible brightness levels (0 = off, 15 = maximum)
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- The display is written one line at a time
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- After the display is selected with `DEV`, writing begins on the top line
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- Writing wraps around and begins at the top again, if more than 5 lines are written
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### Device numbers
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1. Input - Card Reader
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2. Output - Card Punch
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3. Output - Line Printer
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4. Output - Matrix Display
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## MicroCardiograph (microprocessor trainer)
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The MicroCardiograph uses memory-mapped IO.
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### Memory map
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| Address | Used for... |
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|----------|-----------------------------------------------|
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| 00 to 19 | display (5x5) |
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| 1A | pointer to display memory |
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| 1B | keypad: value of latest key pressed |
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| 1C | reserved for future use (bank switching flag) |
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| 1D | initial IP |
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| 1D to FE | free |
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| FF | * ROM (unwriteable) pointer to initial IP |
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\* Not implemented yet
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### Peripherals
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#### Keypad
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The value of the latest keypress on a hex keypad is stored at `$1B`.
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The keypad uses the same layout as the COSMAC VIP (and CHIP-8). The CPU simulator maps those keys onto a Qwerty set:
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`1` `2` `3` `C` = `1` `2` `3` `4`
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`4` `5` `6` `D` = `Q` `W` `E` `R`
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`7` `8` `9` `E` = `A` `S` `D` `F`
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`A` `0` `B` `F` = `Z` `X` `C` `V`
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The arrow keys are also mapped onto the hex keypad:
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` ` `5` ` ` = ` ` `↑` ` `
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`7` `8` `9` = `←` `↓` `→`
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## Assembly language
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ADD $01 ; comments follow a `;`
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ADD $FF ; this is direct addressing
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ADD ($CC) ; this is indirect addressing
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END ; END and NOP don't require operands
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; (the assembler will fill in a default value of 0)
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@subroutine ; create a label
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ADD $01 ; (it must be on the line before the code it names)
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ADD $02
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JMP @subroutine ; use a label as operand
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; the label will be replaced with
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; the address of the label
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#foo $FF ; define a constant
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; (must be defined before it is referenced)
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ADD #foo ; use a constant as an operand
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LDA * ; `*` is a special label referencing the memory address
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; where the current line will be stored after assembly
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- Prefix hexadecimal numbers with `$` (or `0x`)
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- Prefix binary numbers with `0b`
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- Whitespace is ignored |