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holey-bytes/spec.md

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# HoleyBytes ISA Specification
# Bytecode format
- All numbers are encoded little-endian
- There is 256 registers, they are represented by a byte
- Immediate values are 64 bit
- Program is by spec required to be terminated with 12 zero bytes
### Instruction encoding
- Instruction parameters are packed (no alignment)
- [opcode, …parameters…]
### Instruction parameter types
- B = Byte
- D = Doubleword (64 bits)
- H = Halfword (16 bits)
| Name | Size |
|:----:|:--------|
| BBBB | 32 bits |
| BBB | 24 bits |
| BBDH | 96 bits |
| BBD | 80 bits |
| BB | 16 bits |
| BD | 72 bits |
| D | 64 bits |
| N | 0 bits |
# Instructions
- `#n`: register in parameter *n*
- `imm #n`: for immediate in parameter *n*
- `P ← V`: Set register P to value V
- `[x]`: Address x
## No-op
- N type
| Opcode | Name | Action |
|:------:|:----:|:-----------------------------:|
| 0 | UN | Trigger unreachable code trap |
| 1 | NOP | Do nothing |
## Integer binary ops.
- BBB type
- `#0 ← #1 <op> #2`
| Opcode | Name | Action |
|:------:|:----:|:-----------------------:|
| 2 | ADD | Wrapping addition |
| 3 | SUB | Wrapping subtraction |
| 4 | MUL | Wrapping multiplication |
| 5 | AND | Bitand |
| 6 | OR | Bitor |
| 7 | XOR | Bitxor |
| 8 | SL | Unsigned left bitshift |
| 9 | SR | Unsigned right bitshift |
| 10 | SRS | Signed right bitshift |
### Comparsion
| Opcode | Name | Action |
|:------:|:----:|:-------------------:|
| 11 | CMP | Signed comparsion |
| 12 | CMPU | Unsigned comparsion |
#### Comparsion table
| #1 *op* #2 | Result |
|:----------:|:------:|
| < | -1 |
| = | 0 |
| > | 1 |
### Division-remainder
- Type BBBB
- In case of `#3` is zero, the resulting value is all-ones
- `#0 ← #2 ÷ #3`
- `#1 ← #2 % #3`
| Opcode | Name | Action |
|:------:|:----:|:-------------------------------:|
| 13 | DIR | Divide and remainder combinated |
### Negations
- Type BB
- `#0 ← #1 <op> #2`
| Opcode | Name | Action |
|:------:|:----:|:----------------:|
| 14 | NEG | Bit negation |
| 15 | NOT | Logical negation |
## Integer immediate binary ops.
- Type BBD
- `#0 ← #1 <op> imm #2`
| Opcode | Name | Action |
|:------:|:----:|:-----------------------:|
| 16 | ADDI | Wrapping addition |
| 17 | MULI | Wrapping subtraction |
| 18 | ANDI | Bitand |
| 19 | ORI | Bitor |
| 20 | XORI | Bitxor |
| 21 | SLI | Unsigned left bitshift |
| 22 | SRI | Unsigned right bitshift |
| 23 | SRSI | Signed right bitshift |
### Comparsion
- Comparsion is the same as when RRR type
| Opcode | Name | Action |
|:------:|:-----:|:-------------------:|
| 24 | CMPI | Signed comparsion |
| 25 | CMPUI | Unsigned comparsion |
## Register value set / copy
### Copy
- Type BB
- `#0 ← #1`
| Opcode | Name | Action |
|:------:|:----:|:------:|
| 26 | CP | Copy |
### Swap
- Type BB
- Swap #0 and #1
| Opcode | Name | Action |
|:------:|:----:|:------:|
| 27 | SWA | Swap |
### Load immediate
- Type BD
- `#0 ← #1`
| Opcode | Name | Action |
|:------:|:----:|:--------------:|
| 28 | LI | Load immediate |
## Memory operations
- Type BBDH
- If loaded/store value exceeds one register size, continue accessing following registers
### Load / Store
| Opcode | Name | Action |
|:------:|:----:|:---------------------------------------:|
| 29 | LD | `#0 ← [#1 + imm #3], copy imm #4 bytes` |
| 30 | ST | `[#1 + imm #3] ← #0, copy imm #4 bytes` |
## Block copy
- Block copy source and target can overlap
### Memory copy
- Type BBD
| Opcode | Name | Action |
|:------:|:----:|:--------------------------------:|
| 31 | BMC | `[#1] ← [#0], copy imm #2 bytes` |
### Register copy
- Type BBB
- Copy a block a register to another location (again, overflowing to following registers)
| Opcode | Name | Action |
|:------:|:----:|:--------------------------------:|
| 32 | BRC | `#1 ← #0, copy imm #2 registers` |
## Control flow
### Unconditional jump
- Type BBD
| Opcode | Name | Action |
|:------:|:----:|:-------------------------------------------------:|
| 33 | JAL | Save current PC to `#0` and jump at `#1 + imm #2` |
### Conditional jumps
- Type BBD
- Jump at `imm #2` if `#0 <op> #1`
| Opcode | Name | Comparsion |
|:------:|:----:|:------------:|
| 34 | JEQ | = |
| 35 | JNE | ≠ |
| 36 | JLT | < (signed) |
| 37 | JGT | > (signed) |
| 38 | JLTU | < (unsigned) |
| 39 | JGTU | > (unsigned) |
### Environment call
- Type N
| Opcode | Name | Action |
|:------:|:-----:|:-------------------------------------:|
| 40 | ECALL | Cause an trap to the host environment |
## Floating point operations
- Type BBB
- `#0 ← #1 <op> #2`
| Opcode | Name | Action |
|:------:|:----:|:--------------:|
| 41 | ADDF | Addition |
| 42 | SUBF | Subtraction |
| 43 | MULF | Multiplication |
### Division-remainder
- Type BBBB
| Opcode | Name | Action |
|:------:|:----:|:-------------------------:|
| 44 | DIRF | Same as for integer `DIR` |
### Fused Multiply-Add
- Type BBBB
| Opcode | Name | Action |
|:------:|:----:|:---------------------:|
| 45 | FMAF | `#0 ← (#1 * #2) + #3` |
### Negation
- Type BB
| Opcode | Name | Action |
|:------:|:----:|:----------:|
| 46 | NEGF | `#0 ← -#1` |
### Conversion
- Type BB
- Signed
- `#0 ← #1 as _`
| Opcode | Name | Action |
|:------:|:----:|:------------:|
| 47 | ITF | Int to Float |
| 48 | FTI | Float to Int |
## Floating point immediate operations
- Type BBD
- `#0 ← #1 <op> imm #2`
| Opcode | Name | Action |
|:------:|:-----:|:--------------:|
| 49 | ADDFI | Addition |
| 50 | MULFI | Multiplication |
# Registers
- There is 255 registers + one zero register (with index 0)
- Reading from zero register yields zero
- Writing to zero register is a no-op
# Memory
- Addresses are 64 bit
- Memory implementation is arbitrary
- In case of accessing invalid address:
- Program shall trap (LoadAccessEx, StoreAccessEx) with parameter of accessed address
- Value of register when trapped is undefined
## Recommendations
- Leave address `0x0` as invalid
- If paging used:
- Leave first page invalid
- Pages should be at least 4 KiB
# Program execution
- The way of program execution is implementation defined
- The order of instruction is arbitrary, as long all observable
effects are applied in the program's order
# Program validation
- Invalid program should cause runtime error:
- The form of error is arbitrary. Can be a trap or an interpreter-specified error
- It shall not be handleable from within the program
- Executing invalid opcode should trap
- Program can be validaded either before execution or when executing
# Traps
Program should at least implement these traps:
- Environment call
- Invalid instruction exception
- Load address exception
- Store address exception
and executing environment should be able to get information about them,
like the opcode of invalid instruction or attempted address to load/store.
Details about these are left as an implementation detail.
# Assembly
HoleyBytes assembly format is not defined, this is just a weak description
of `hbasm` syntax.
- Opcode names correspond to specified opcode names, lowercase (`nop`)
- Parameters are separated by comma (`addi r0, r0, 1`)
- Instructions are separated by either line feed or semicolon
- Registers are represented by `r` followed by the number (`r10`)
- Labels are defined by label name followed with colon (`loop:`)
- Labels are references simply by their name (`print`)
- Immediates are entered plainly. Negative numbers supported.
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