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Add MiMa specification

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# mima-tools # mima-tools
## MiMa specification
### General
The MiMa uses words of 24 bits and addresses of 20 bits.
Each step, the MiMa fetches the value at the address stored in the
`IAR`, interprets it as an instruction and executes it. If the
instruction does not explicitly modify the `IAR`, it is incremented by
1 automatically.
During execution, multiple situations can be encountered where
execution should not be continued, for example:
* After the `HALT` instruction was executed
* The value stored at the current address of the `IAR` cannot be
decoded to a valid instruction
* The instruction pointer is at value `0xFFFFF` and the instruction
does not modify the `IAR`
In those cases, a MiMa emulator should stop execution and show a
suitable error message explaining why execution could not continue.
### Instructions
An instruction has one of the following forms:
```
Small opcode:
+----+ +-----------------------+
| SO | | Value/Address |
+----+ +-----------------------+
23 20 19 0
Large opcode:
+----+ +----+ +----------------+
| F | | LO | | Value |
+----+ +----+ +----------------+
23 20 19 16 15 0
```
Small opcodes can range from `0` to `E` and have an address or 20-bit
value as argument. Large opcodes can range from `F0` to `FF` and have,
if at all, a 16-bit value as argument.
### Registers
| Name | Size (bits) | Function |
|-------|-------------|------------------------------|
| `IAR` | 20 | Instruction Address Register |
| `ACC` | 24 | Accumulator |
| `RA` | 20 | Return Address |
| `SP` | 20 | Stack Pointer |
| `FP` | 20 | Frame Pointer |
### Opcodes
| Opcode | Name | Function | Notes |
|--------|---------------------------------------------|--------------------------------|------------------------------------|
| `0` | `LDC c` (load constant) | `c -> ACC` | Upper 4 bits of `ACC` are set to 0 |
| `1` | `LDV a` (load value) | `<a> -> ACC` | |
| `2` | `STV a` (store value) | `ACC -> <a>` | |
| `3` | `ADD a` | `ACC + <a> -> ACC` | |
| `4` | `AND a` | `ACC and <a> -> ACC` | Bitwise operation |
| `5` | `OR a` | `ACC or <a> -> ACC` | Bitwise operation |
| `6` | `XOR a` | `ACC xor <a> -> ACC` | Bitwise operation |
| `7` | `EQL a` (equal) | `(ACC == <a> ? -1 : 0) -> ACC` | |
| `8` | `JMP a` (jump) | `a -> IAR` | |
| `9` | `JMN a` (jump if negative) | `if (ACC < 0) {a -> IAR}` | |
| `A` | `LDIV a` (load indirect value) | `<<a>> -> ACC` | Upper 4 bits of `<a>` are ignored |
| `B` | `STIV a` (store indirect value) | `ACC -> <<a>>` | Upper 4 bits of `<a>` are ignored |
| `C` | `CALL a` | `IAR -> RA; JMP a` | |
| `D` | `LDVR d` (load value with relative offset) | `<SP + d> -> ACC` | |
| `E` | `STVR d` (store value with relative offset) | `ACC -> <SP + d>` | |
| `F0` | `HALT` | Halt execution | |
| `F1` | `NOT` | `not ACC -> ACC` | Bitwise operation |
| `F2` | `RAR` (rotate ACC right) | `ACC >> 1 -> ACC` | See below |
| `F3` | `RET` (return) | `RA -> IAR` | |
| `F4` | `LDSP` (load from SP) | `SP -> ACC` | |
| `F5` | `STSP` (store to SP) | `ACC -> SP` | |
| `F6` | `LDFP` (load from FP) | `FP -> ACC` | |
| `F7` | `STFP` (store to FP) | `ACC -> FP` | |
| `F8` | `LDRA` (load from RA) | `RA -> ACC` | |
| `F9` | `STRA` (store to RA) | `ACC -> RA` | |
| `FA` | `ADC c` (add constant) | `ACC + c -> ACC` | See below |
- `RAR` shifts all bits in the `ACC` right by one. The rightmost bit wraps around to the leftmost position.
- `ADC c` interprets bits 15-0 as a signed integer, whose value is then added to the `ACC`'s current value.
## File format ## File format
All tools share a common file format, which basically just contains All tools share a common file format with extension `.mima`. It
the MiMa's initial memory state. Its file extension is `.mima`. contains the whole execution state of a MiMa, meaning the contents of
its memory and all its registers.
A MiMa operates on words of 24 bits, so the file is split up into The file is split up into blocks of 3 bytes, which form MiMa
blocks of 3 bytes, written directly one after the other with nothing words. The bytes within a word are ordered from most to least
in-between. The bytes within one 3-byte block are ordered from most significant.
significant to least significant.
The file contains no metadata. Opcodes are the same as specified in The values of registers which are only 20 bits long are stored in the
the lecture. The first block is at address 0. MiMa's execution starts lower 20 bits of a MiMa word, and the remaining bits 23-20 are filled
at address 0 (i. e. the first block). with zeroes, like so:
```
+----+ +-----------------------+
| 0 | | 20-bit register value |
+----+ +-----------------------+
23 20 19 0
```
The registers and memory are stored as follows:
| Word | Content |
|---------------+-------------|
| 0 | `IR` |
| 1 | `ACC` |
| 2 | `RA` |
| 3 | `SP` |
| 4 | `FP` |
| starting at 6 | Memory dump |
The memory dump contains the words of the MiMa's memory, written in
increasing order directly one after the other with nothing
in-between. The dump always starts at address `0x00000`, but may end
before it reaches address `0xFFFFF`. When reading a dump, all
unspecified values are to be intialized as `0x000000`.
A `.mima` file must always be a multiple of 3 bytes long. It must
always be at least 15 bytes long (contains all register values).
## Programs ## Programs
@ -20,6 +134,8 @@ at address 0 (i. e. the first block).
This program can load and run `.mima` files. This program can load and run `.mima` files.
It currently does not follow the specification above.
``` ```
$ mima-run --help $ mima-run --help
Usage: mima-run INFILE [-n|--steps N] [-d|--dump OUTFILE] [-q|--quiet] Usage: mima-run INFILE [-n|--steps N] [-d|--dump OUTFILE] [-q|--quiet]