mirror of
https://github.com/pnx/m16vm
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113 lines
3.9 KiB
Markdown
113 lines
3.9 KiB
Markdown
m16vm - 16 bit processor virtual machine
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========================================
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This is a virtual machine for a RISC-processor designed for educational purposes.
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The design is taken from the real world and tries to mimic existing RISC-architectures. So it should in theory be possible to construct actual hardware based on the instruction set. Maybe not build a transistor-chip (it's hard, unless you know someone working at Intel/Amd or something) but by putting together a bunch of gate-chips on a breadboard.
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The instruction set is by no means designed to be "fast" or "optimal" but focus more on being easy to understand and modify/play with.
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Specification
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-------------
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The cpu has 16 general purpose registers, r0 - r15. Each register is 16-bit.
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Memory addresses are aligned by 2 bytes (16 bit) and the machine has a total of `65536` bytes of memory (per default, can be changed at compile time).
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instruction set overview
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------------------------
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There are 16 different instructions.
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| opcode | Name | Type | Description |
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|--------|------|------|-----------------------------------------|
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| 0000 | noop | \- | No operation |
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| 0001 | add | R | Addition |
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| 0010 | movl | I | move value to register (lowest 8-bits) |
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| 0011 | movh | I | move value to register (highest 8-bits) |
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| 0100 | ld | RI | load word |
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| 0101 | sw | RI | store word |
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| 0110 | beq | RI | Branch on equal |
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| 0111 | jmp | J | Jump |
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| 1000 | jr | I | Jump (register) |
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| 1001 | \- | \- | Reserved |
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| 1010 | \- | \- | Reserved |
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| 1011 | \- | \- | Reserved |
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| 1100 | \- | \- | Reserved |
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| 1101 | \- | \- | Reserved |
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| 1110 | \- | \- | Reserved |
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| 1111 | int | I | Interrupt |
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NOTE: Subtraction can be implemented via `add` and negative register values, so no special opcode is needed.
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Instruction set format
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----------------------
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Register operation (R):
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```
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opcode (4) | rs (4) | r0 (4) | r1 (4)
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r0,r1 - Operand registers.
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rs - save register
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```
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Register operation offset (RI):
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```
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opcode (4) | rs (4) | r0 (4) | offset (signed 4)
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r0 - Operand registers.
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offset - constant offset from r0 value.
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rs - save register
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```
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Constant operation format (I):
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```
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opcode (4) | reg (4) | data (signed 8)
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reg - Register
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data - Constant data to insert into reg.
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```
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Jump format (J):
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```
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opcode (4) | addr (12)
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```
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Interrupts
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----------
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There can be a total of `16` Interrupts and the instruction uses the `I-format`.
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The interrupt number is stored in `reg` (note that for interrupts this is a constant and not a register).
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The `data` field can be used as an argument to the interrupt.
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### Interrupt functions overview
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| Number | Name | data | Description |
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|-------:|-----------|--------|----------------------------------------------------------|
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| 10 | I/O Write | format | Outputs a value to the screen, value are stored in `r15` |
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### I/O Write - int 10
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The formatting of the output can be controlled by the number in `data` as follows:
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| Argument | Datatype |
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|----------|-------------------------|
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| 0 | Integer (16 bit signed) |
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| 1 | Integer (8 bit signed) |
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| 2 | Character |
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Example programs.
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-----------------
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in the `programs` directory, there is some example programs to run.
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Author
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======
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Henrik Hautakoski - [henrik.hautakoski@gmail.com](mailto:henrik.hautakoski@gmail.com)
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