HoleyBytes ISA Specification
Bytecode format
- Holey Bytes program should start with following magic:
[0xAB, 0x1E, 0x0B]
- 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 |
| BBW |
48 bits |
| BB |
16 bits |
| BD |
72 bits |
| D |
64 bits |
| N |
0 bits |
Instructions
#n: register in parameter nimm #n: for immediate in parameter nP ← V: Set register P to value V[x]: Address x
Program execution control
| Opcode |
Name |
Action |
| 0 |
UN |
Trigger unreachable code trap |
| 1 |
TX |
Terminate execution |
| 2 |
NOP |
Do nothing |
Integer binary ops.
| Opcode |
Name |
Action |
| 3 |
ADD |
Wrapping addition |
| 4 |
SUB |
Wrapping subtraction |
| 5 |
MUL |
Wrapping multiplication |
| 6 |
AND |
Bitand |
| 7 |
OR |
Bitor |
| 8 |
XOR |
Bitxor |
| 9 |
SL |
Unsigned left bitshift |
| 10 |
SR |
Unsigned right bitshift |
| 11 |
SRS |
Signed right bitshift |
Comparsion
| Opcode |
Name |
Action |
| 12 |
CMP |
Signed comparsion |
| 13 |
CMPU |
Unsigned comparsion |
Comparsion table
| #1 op #2 |
Result |
| < |
0 |
| = |
1 |
| > |
2 |
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 |
| 14 |
DIR |
Divide and remainder combinated |
Negations
| Opcode |
Name |
Action |
| 15 |
NEG |
Bit negation |
| 16 |
NOT |
Logical negation |
Integer immediate binary ops.
- Type BBD
#0 ← #1 <op> imm #2
| Opcode |
Name |
Action |
| 17 |
ADDI |
Wrapping addition |
| 18 |
MULI |
Wrapping subtraction |
| 19 |
ANDI |
Bitand |
| 20 |
ORI |
Bitor |
| 21 |
XORI |
Bitxor |
Bitshifts
- Type BBW
| Opcode |
Name |
Action |
| 22 |
SLI |
Unsigned left bitshift |
| 23 |
SRI |
Unsigned right bitshift |
| 24 |
SRSI |
Signed right bitshift |
Comparsion
- Comparsion is the same as when RRR type
| Opcode |
Name |
Action |
| 25 |
CMPI |
Signed comparsion |
| 26 |
CMPUI |
Unsigned comparsion |
Register value set / copy
Copy
| Opcode |
Name |
Action |
| 27 |
CP |
Copy |
Swap
- Type BB
- Swap #0 and #1
- Zero register rules:
- Both: no-op
- One: Copy zero to the non-zero register
| Opcode |
Name |
Action |
| 28 |
SWA |
Swap |
Load immediate
| Opcode |
Name |
Action |
| 29 |
LI |
Load immediate |
Load relative address
- Type BBW
| Opcode |
Name |
Action |
| 30 |
LRA |
#0 ← #1 + imm #2 + PC |
Memory operations
- Type BBDH
- If loaded/store value exceeds one register size, continue accessing following registers
Load / Store
| Opcode |
Name |
Action |
| 31 |
LD |
#0 ← [#1 + imm #2], copy imm #3 bytes |
| 32 |
ST |
[#1 + imm #2] ← #0, copy imm #3 bytes |
PC relative Load / Store
- Type BBDW
| Opcode |
Name |
Action |
| 33 |
LDR |
#0 ← [#1 + imm #2 + PC], copy imm #3 bytes |
| 34 |
STR |
[#1 + imm #2 + PC] ← #0, copy imm #3 bytes |
Block copy
- Block copy source and target can overlap
Memory copy
| Opcode |
Name |
Action |
| 35 |
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 |
| 36 |
BRC |
#1 ← #0, copy imm #2 registers |
Control flow
Unconditional jump
- Type D
| Opcode |
Name |
Action |
| 37 |
JMP |
Unconditional, non-linking jump |
| 38 |
JMPR |
Jump at address relative to program counter |
Unconditional linking jump
| Opcode |
Name |
Action |
| 39 |
JAL |
Save PC past JAL to #0 and jump at #1 + imm #2 |
| 40 |
JALR |
Save PC past JAL to #0 and jump at #1 + imm #2 + PC |
Conditional jumps
- Type BBH
- Jump at
PC + imm #2 if #0 <op> #1
| Opcode |
Name |
Comparsion |
| 41 |
JEQ |
= |
| 42 |
JNE |
≠ |
| 43 |
JLT |
< (signed) |
| 44 |
JGT |
> (signed) |
| 45 |
JLTU |
< (unsigned) |
| 46 |
JGTU |
> (unsigned) |
Environment call
| Opcode |
Name |
Action |
| 47 |
ECALL |
Cause an trap to the host environment |
Floating point operations
| Opcode |
Name |
Action |
| 48 |
ADDF |
Addition |
| 49 |
SUBF |
Subtraction |
| 50 |
MULF |
Multiplication |
Division-remainder
| Opcode |
Name |
Action |
| 51 |
DIRF |
Same as for integer DIR |
Fused Multiply-Add
| Opcode |
Name |
Action |
| 52 |
FMAF |
#0 ← (#1 * #2) + #3 |
Negation
- Type BB
| Opcode |
Name |
Action |
| 53 |
NEGF |
#0 ← -#1 |
Conversion
- Type BB
- Signed
#0 ← #1 as _
| Opcode |
Name |
Action |
| 54 |
ITF |
Int to Float |
| 55 |
FTI |
Float to Int |
Floating point immediate operations
- Type BBD
#0 ← #1 <op> imm #2
| Opcode |
Name |
Action |
| 56 |
ADDFI |
Addition |
| 57 |
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
- Program should be in the same address space as all other data
- Memory implementation is arbitrary
- Address
0x0 may or may not be valid. Count with compilers
considering it invalid!
- In case of accessing invalid address:
- Program shall trap (LoadAccessEx, StoreAccessEx) with parameter of accessed address
- Value of register when trapped is undefined
Recommendations
- 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 execution is arbitrary, as long all effects are obervable
in the way as program was executed literally, in 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
- Unreachable instruction
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.
