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