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# [DRAFT] Specification for the _Cardiograph Architecture_
## CPU
### Registers
There are four 8-bit registers:
1. **A**, the accumulator (and the only general-purpose register)
2. **IP**, the instruction pointer (aka program counter)
3. **IOD**, the ID of the current I/O device
3. **Status**
#### Status register
The *high byte* holds the state of the four Sense Switches. (TODO: is this easy enough to do in hardware?)
The *low byte* holds four flags:
**O**verflow, **N**egative, **Z**ero, and **C**arry.
These are all addressed by number:*
| S1 | S2 | S3 | S4 | | O | N | Z | C |
|----|----|----|----|-|----|----|----|----|
| 80 | 40 | 20 | 10 | | 08 | 04 | 02 | 01 |
* (Because the core instruction set doesn't include bitwise operations)
### Instruction set
- Instructions are two bytes long:
one byte for the opcode, one for the operand
<mark>TODO: revise this based on note dated 2023-09-24</mark>
```GGMM IIII``` - **G**roup, **M**ode, **I**nstruction
| lo ↓ / hi → | 0 (G0, M0) | 5 (G1, M1) | 6 (G1, M2) | 9 (G2, M1) | A (G2, M2) | F (G3, M3) |
|-------------|------------|------------|------------|------------|------------|------------|
| **0** | END | LDA # | LDA ind | DEV # | DEV ind | |
| **1** | NOP | STO # | STO ind | INP # | INP ind | |
| **2** | | ADD # | ADD ind | OUT # | OUT ind | |
| **3** | | SUB # | SUB ind | FED | FED | |
| **4** | | JMP # | JMP ind | | | |
| **5** | | JEQ # | JEQ ind | | | |
| **6** | | JFL # | JFL ind | | | |
| **7** | | FTG # | FTG ind | | | |
| | | | | | | |
| **8** | | MUL # | MUL ind | | | RSL A |
| **9** | | DIV # | DIV ind | | | RSR A |
| **A** | | JLT # | JLT # | | | ASL A |
| **B** | | JGT # | JGT # | | | ASR A |
| **C** | | NOT # | NOT # | | | |
| **D** | | AND # | AND # | | | |
| **E** | | OR # | OR # | | | |
| **F** | | XOR # | XOR # | | | |
TODO: assess JMPs vs. HOPs
- RSL/RSR: Ring Shift Left/Right
- JLT/JGT: Jump Less/Greater Than
- DEV: select IO device
- FED: "feed" - line feed / end of card
TODO: format/document better:
1. core computational operations: low nibbles of 0x, 5x, 6x
2. arithmetic extension (optional): MUL, DIV
3. IO extension (optional): 9x, Ax
4. bitwise arithmetic extension (optional): NOT, AND, OR, XOR and RSL, RSR, ASL, ASR
5. control flow extension (optional): JLT, JGT
- The mainframe system implements at least 1, 2, and 3
- The microprocessor trainer implements 1
- (see note dated 2023-09-24)
### Connections (pinout)
<mark>TBC</mark>
| name | in/out? | description |
|-----------|---------|---------------|
| RST | in | *reset* |
| VCC | in | *power* |
| GND | in | *ground* |
| CLK | in | *clock* |
| A0 - A7 | out | *address bus* |
| D0 - D7 | out | *data bus* |
| ABE | out | *address bus enable*: <br> low when the CPU is using the address bus |
| DBE | out | *data bus enable*: <br> low when the CPU is using the data bus |
| WAIT | in | *wait* — when pulled low, <br> the current operation is completed <br> and then execution pauses |
| /RD | out | TODO |
| /WR | out | |
| M/IO | out | |
### Start-up
<mark>TODO: see if this makes sense for the mainframe </mark>
When starting up, the CPU executes a `JMP $FF`.
Put differently: it starts executing instructions at the address contained in `$FF`.
<mark>TODO: currently the simulator doesn't actually do this</mark>

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# Assembly language
## Syntax
ADD $01 ; comments follow a `;`
ADD $FF ; this is direct addressing
ADD ($CC) ; this is indirect addressing
END ; END and NOP don't require operands
; (the assembler will fill in a default value of 0)
@subroutine ; create a label
ADD $01 ; (it must be on the line before the code it names)
ADD $02
JMP @subroutine ; use a label as operand
; the label will be replaced with
; the address of the label
#foo $FF ; define a constant
; (must be defined before it is referenced)
ADD #foo ; use a constant as an operand
LDA * ; `*` is a special label referencing the memory address
; where the current line will be stored after assembly
- Prefix hexadecimal numbers with `$` (or `0x`)
- Prefix binary numbers with `0b`
- Whitespace is ignored

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## Cardiograph Mark I (mainframe)
The components of a Mark I are:
- an CG 101 Central Processing Unit
- an CG 102 Core Memory Unit
- an CG 103 Print-Key-Punch
- an CG 104 Matrix Display
### Console
<mark>TBC TBC TBC</mark>
The console is equipped with:
- Power switch
- Load button
- Run button
- Run Single Step button
- Halt button
- Memory Read button
- Memory Read Next button
- Memory Write button
- Memory Write Next button
- 4 Sense Switches
- 8 Accumulator lights
- 8 Address lights
- 8 Data lights
- 8 Instruction Pointer lights (<mark>review IP size?</mark>)
- 8 Status Register lights
## 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) |

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# [DRAFT] Design for the _MicroCardiograph_ microprocessor trainer
The MicroCardiograph uses memory-mapped IO.
## Memory map
| Address | Used for... |
|----------|-----------------------------------------------|
| 00 to 19 | display (5x5) |
| 1A | pointer to display memory |
| 1B | keypad: value of latest key pressed |
| 1C | reserved for future use (bank switching flag) |
| 1D | initial IP |
| 1D to FE | free |
| FF | * ROM (unwriteable) pointer to initial IP |
\* Not implemented yet
## Peripherals
### Keypad
The value of the latest keypress on a hex keypad is stored at `$1B`.
The keypad uses the same layout as the COSMAC VIP (and CHIP-8). The CPU simulator maps those keys onto a Qwerty set:
`1` `2` `3` `C`   =   `1` `2` `3` `4`
`4` `5` `6` `D`   =   `Q` `W` `E` `R`
`7` `8` `9` `E`   =   `A` `S` `D` `F`
`A` `0` `B` `F`   =   `Z` `X` `C` `V`
The arrow keys are also mapped onto the hex keypad:
` ` `5` ` `   =   ` ` `↑` ` `
`7` `8` `9`   =   `←` `↓` `→`
### Keypad as monitor input
TODO

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_[Micro ElectroCardiograph (µECG)](micro/readme-micro.md)_ is a simulator for the Micro Cardiograph.
## CPU
## Documentation
### Registers
There are four 8-bit registers:
1. **A**, the accumulator (and the only general-purpose register)
2. **IP**, the instruction pointer (aka program counter)
3. **IOD**, the ID of the current I/O device
3. **Status**
#### Status register
The *high byte* holds the state of the four Sense Switches. (TODO: is this easy enough to do in hardware?)
The *low byte* holds four flags:
**O**verflow, **N**egative, **Z**ero, and **C**arry.
These are all addressed by number:*
| S1 | S2 | S3 | S4 | | O | N | Z | C |
|----|----|----|----|-|----|----|----|----|
| 80 | 40 | 20 | 10 | | 08 | 04 | 02 | 01 |
* (Because the core instruction set doesn't include bitwise operations)
### Instruction set
- Instructions are two bytes long:
one byte for the opcode, one for the operand
<mark>TODO: revise this based on note dated 2023-09-24</mark>
```GGMM IIII``` - **G**roup, **M**ode, **I**nstruction
| lo ↓ / hi → | 0 (G0, M0) | 5 (G1, M1) | 6 (G1, M2) | 9 (G2, M1) | A (G2, M2) | F (G3, M3) |
|-------------|------------|------------|------------|------------|------------|------------|
| **0** | END | LDA # | LDA ind | DEV # | DEV ind | |
| **1** | NOP | STO # | STO ind | INP # | INP ind | |
| **2** | | ADD # | ADD ind | OUT # | OUT ind | |
| **3** | | SUB # | SUB ind | FED | FED | |
| **4** | | JMP # | JMP ind | | | |
| **5** | | JEQ # | JEQ ind | | | |
| **6** | | JFL # | JFL ind | | | |
| **7** | | FTG # | FTG ind | | | |
| | | | | | | |
| **8** | | MUL # | MUL ind | | | RSL A |
| **9** | | DIV # | DIV ind | | | RSR A |
| **A** | | JLT # | JLT # | | | ASL A |
| **B** | | JGT # | JGT # | | | ASR A |
| **C** | | NOT # | NOT # | | | |
| **D** | | AND # | AND # | | | |
| **E** | | OR # | OR # | | | |
| **F** | | XOR # | XOR # | | | |
TODO: assess JMPs vs. HOPs
- RSL/RSR: Ring Shift Left/Right
- JLT/JGT: Jump Less/Greater Than
- DEV: select IO device
- FED: "feed" - line feed / end of card
TODO: format/document better:
1. core computational operations: low nibbles of 0x, 5x, 6x
2. arithmetic extension (optional): MUL, DIV
3. IO extension (optional): 9x, Ax
4. bitwise arithmetic extension (optional): NOT, AND, OR, XOR and RSL, RSR, ASL, ASR
5. control flow extension (optional): JLT, JGT
- The mainframe system implements at least 1, 2, and 3
- The microprocessor trainer implements 1
- (see note dated 2023-09-24)
### Connections (pinout)
<mark>TBC</mark>
| name | in/out? | description |
|-----------|---------|---------------|
| RST | in | *reset* |
| VCC | in | *power* |
| GND | in | *ground* |
| CLK | in | *clock* |
| A0 - A7 | out | *address bus* |
| D0 - D7 | out | *data bus* |
| ABE | out | *address bus enable*: <br> low when the CPU is using the address bus |
| DBE | out | *data bus enable*: <br> low when the CPU is using the data bus |
| WAIT | in | *wait* — when pulled low, <br> the current operation is completed <br> and then execution pauses |
| /RD | out | TODO |
| /WR | out | |
| M/IO | out | |
### Start-up
<mark>TODO: see if this makes sense for the mainframe </mark>
When starting up, the CPU executes a `JMP $FF`.
Put differently: it starts executing instructions at the address contained in `$FF`.
<mark>TODO: currently the simulator doesn't actually do this</mark>
## Cardiograph Mark I (mainframe)
The components of a Mark I are:
- an CG 101 Central Processing Unit
- an CG 102 Core Memory Unit
- an CG 103 Print-Key-Punch
- an CG 104 Matrix Display
### Console
<mark>TBC TBC TBC</mark>
The console is equipped with:
- Power switch
- Load button
- Run button
- Run Single Step button
- Halt button
- Memory Read button
- Memory Read Next button
- Memory Write button
- Memory Write Next button
- 4 Sense Switches
- 8 Accumulator lights
- 8 Address lights
- 8 Data lights
- 8 Instruction Pointer lights (<mark>review IP size?</mark>)
- 8 Status Register lights
## 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) |
## MicroCardiograph (microprocessor trainer)
The MicroCardiograph uses memory-mapped IO.
### Memory map
| Address | Used for... |
|----------|-----------------------------------------------|
| 00 to 19 | display (5x5) |
| 1A | pointer to display memory |
| 1B | keypad: value of latest key pressed |
| 1C | reserved for future use (bank switching flag) |
| 1D | initial IP |
| 1D to FE | free |
| FF | * ROM (unwriteable) pointer to initial IP |
\* Not implemented yet
### Peripherals
#### Keypad
The value of the latest keypress on a hex keypad is stored at `$1B`.
The keypad uses the same layout as the COSMAC VIP (and CHIP-8). The CPU simulator maps those keys onto a Qwerty set:
`1` `2` `3` `C`   =   `1` `2` `3` `4`
`4` `5` `6` `D`   =   `Q` `W` `E` `R`
`7` `8` `9` `E`   =   `A` `S` `D` `F`
`A` `0` `B` `F`   =   `Z` `X` `C` `V`
The arrow keys are also mapped onto the hex keypad:
` ` `5` ` `   =   ` ` `↑` ` `
`7` `8` `9`   =   `←` `↓` `→`
## Assembly language
ADD $01 ; comments follow a `;`
ADD $FF ; this is direct addressing
ADD ($CC) ; this is indirect addressing
END ; END and NOP don't require operands
; (the assembler will fill in a default value of 0)
@subroutine ; create a label
ADD $01 ; (it must be on the line before the code it names)
ADD $02
JMP @subroutine ; use a label as operand
; the label will be replaced with
; the address of the label
#foo $FF ; define a constant
; (must be defined before it is referenced)
ADD #foo ; use a constant as an operand
LDA * ; `*` is a special label referencing the memory address
; where the current line will be stored after assembly
- Prefix hexadecimal numbers with `$` (or `0x`)
- Prefix binary numbers with `0b`
- Whitespace is ignored
- [Specification for the Cardiograph Architecture](docs/architecture-specification.md)
- [Design for the mainframe computer](docs/mainframe-design.md)
- [Design for the micro computer](docs/micro-design.md)
- [Assembly Language](docs/assembly-language.md)