holey-bytes/spec.md

HoleyBytes ISA Specification

Bytecode format

• Image format is not specified, though ELF is recommended
• All numbers are encoded little-endian
• There is 256 registers, they are represented by a byte
• Immediate values are 8, 16, 32 or 64 bit

Instruction encoding

• Instruction operands are packed (no alignment)
• [opcode, operand 0, operand 1, …]

Instruction parameter types

• R: Register (8 bits)
• Relative program-counter offset immediates:
  • O: 32 bit (Si32)
  • P: 16 bit (Si16)
• Immediates:
  • B: Byte, 8 bit (Xi8)
  • H: Half-word, 16 bit (Xi16)
  • W: Word, 32 bit (Xi32)
  • D: Double-word, 64 bit (Xi64)
• A: Absolute address immediate, 64 bit (Ui64)

Types

• Sin: Signed integer of size n bits (Si8, Si16, Si32, Si64)
• Uin: Unsigned integer of size n bits (Ui8, Ui16, Ui32, Ui64)
• Xin: Sign-agnostic integer of size n bits (Xi8, Xi16, Xi32, Xi64)
• Fln: Floating point number of size n bits (Fl32, Fl64)

Behaviour

• There is only one type of register, a general-purpose one. Used for both integers and floats.
• Integer operations are wrapping, including signed numbers
  • Bitshifts are truncating
• Two's complement
• Floats as specified by IEEE 754
• Execution model is implementation defined as long all observable effects are performed in correct order

Relative addressing

Relative addresses are computed from address of the first byte of offset in the code. Not from the beginning of current or following instruction.

Zero register

• Register 0
• Cannot be clobbered
  • Write is no-op
• Load always yields 0

Rounding modes

Rounding mode	Value
To nearest, ties to even	0b00
Towards 0 (truncate)	0b01
Towards +∞ (up)	0b10
Towards -∞ (down)	0b11

• Remaining values in the byte traps with invalid operand exception

Memory

• Memory implementation is implementation-defined
• Zero address (0x0) is considered invalid

Traps

• Environment call
• Environment breakpoint

Program counter goes to the following instruction

Exceptions

• Memory access fault
• Invalid operand
• Unknown opcode

Program counter stays on the currently executed instruction

Instructions

• #n: register in parameter n
• $n: for immediate in parameter n
• #P ← V: Set register P to value V
• [x]: Address x
• XY: X bytes from location Y
• pc: Program counter
• <XYZ>: Placeholder
• Type(X): Cast

Program execution control

• Type N

Opcode	Mnemonic	Action
0x00	UN	Throw unreachable code exception
0x01	TX	Terminate execution (eg. on end of program)
0x02	NOP	Do nothing

Binary register-register ops

• Type RRR
• Action: #0 ← #1 <OP> #2

Addition (+)

Opcode	Mnemonic	Type
0x03	ADD8	Xi8
0x04	ADD16	Xi16
0x05	ADD32	Xi32
0x06	ADD64	Xi64

Subtraction (-)

Opcode	Mnemonic	Type
0x07	SUB8	Xi8
0x08	SUB16	Xi16
0x09	SUB32	Xi32
0x0A	SUB64	Xi64

Multiplication (*)

Opcode	Mnemonic	Type
0x0B	MUL8	Xi8
0x0C	MUL16	Xi16
0x0D	MUL32	Xi32
0x0E	MUL64	Xi64

Bitwise ops (type: Xi64)

Opcode	Mnemonic	Operation
0x0F	AND	Conjunction (&)
0x10	OR	Disjunction (|)
0x11	XOR	Non-equivalence (^)

Unsigned left bitshift (<<)

Opcode	Mnemonic	Type
0x12	SLU8	Ui8
0x13	SLU16	Ui16
0x14	SLU32	Ui32
0x15	SLU64	Ui64

Unsigned right bitshift (>>)

Opcode	Mnemonic	Type
0x16	SRU8	Ui8
0x17	SRU16	Ui16
0x18	SRU32	Ui32
0x19	SRU64	Ui64

Signed right bitshift (>>)

Opcode	Mnemonic	Type
0x1A	SRS8	Si8
0x1B	SRS16	Si16
0x1C	SRS32	Si32
0x1D	SRS64	Si64

Comparsion

• Compares two numbers, saves result to register
• Operation: #0 ← #1 <=> #2

Ordering	Number
<	-1
=	0
>	1

Opcode	Mnemonic	Type
0x1E	CMPU	Ui64
0x1F	CMPS	Si64

Merged divide-remainder

• Type RRRR

• Operation:

  • #0 ← #2 / #3
  • #1 ← #2 % #3

• If dividing by zero:

  • #0 ← Ui64(-1)
  • #1 ← #2

Opcode	Mnemonic	Type
0x20	DIRU8	Ui8
0x21	DIRU16	Ui16
0x22	DIRU32	Ui32
0x23	DIRU64	Ui64
0x24	DIRS8	Si8
0x25	DIRS16	Si16
0x26	DIRS32	Si32
0x27	DIRS64	Si64

Unary register operations (type: Xi64)

• Type: RR
• Operation: #0 ← <OP> #1

Opcode	Mnemonic	Operation
0x28	NEG	Bitwise complement (~)
0x29	NOT	Logical negation (!)

Sign extensions

• Operation: #0 ← Si64(#1)

Opcode	Mnemonic	Source type
0x2A	SXT8	Si8
0x2B	SXT16	Si16
0x2C	SXT32	Si32

Binary register-immediate operations

• Type: RR<IMM>
• Operation: #0 ← #1 <OP> $2

Addition (+)

Opcode	Mnemonic	Type
0x2D	ADDI8	Xi8
0x2E	ADDI16	Xi16
0x2F	ADDI32	Xi32
0x30	ADDI64	Xi64

Multiplication (*)

Opcode	Mnemonic	Type
0x31	MULI8	Xi8
0x32	MULI16	Xi16
0x33	MULI32	Xi32
0x34	MULI64	Xi64

Bitwise ops (type: Xi64)

Opcode	Mnemonic	Operation
0x35	ANDI	Conjunction (&)
0x36	ORI	Disjunction (|)
0x37	XORI	Non-equivalence (^)

Register-immediate bitshifts

• Type: RRB
• Operation: #0 ← #1 <OP> $2

Unsigned left bitshift (<<)

Opcode	Mnemonic	Type
0x38	SLUI8	Ui8
0x39	SLUI16	Ui16
0x3A	SLUI32	Ui32
0x3B	SLUI64	Ui64

Unsigned right bitshift (>>)

Opcode	Mnemonic	Type
0x3C	SRUI8	Ui8
0x3D	SRUI16	Ui16
0x3E	SRUI32	Ui32
0x3F	SRUI64	Ui64

Signed right bitshift (>>)

Opcode	Mnemonic	Type
0x40	SRSI8	Si8
0x41	SRSI16	Si16
0x42	SRSI32	Si32
0x43	SRSI64	Si64

Comparsion

• Compares two numbers, saves result to register
• Operation: #0 ← #1 <=> $2
• Comparsion table same for register-register one

Opcode	Mnemonic	Type
0x44	CMPUI	Ui64
0x45	CMPSI	Si64

Register copies

• Type: RR

Opcode	Mnemonic	Operation
0x46	CP	Copy register value (#0 ← #1)
0x47	SWA	Swap register values (#0 ⇆ #1)

Load immediate

• Load immediate value from code to register
• Type: R<IMM>
• Operation: #0 ← $1

Opcode	Mnemonic	Type
0x48	LI8	Xi8
0x49	LI16	Xi16
0x4A	Li32	Xi32
0x4B	Li64	Xi64

Load relative address

• Compute value from program counter, register value and offset
• Type: RRO
• Operation: #0 ← pc + #1 + $2

Opcode	Mnemonic
0x4C	LRA

Memory access operations

• Immediate $3 specifies size
• If size is greater than register size, it overflows to adjecent register (eg. copying 16 bytes to register r1 copies first 8 bytes to it and the remaining to r2)

Absolute addressing

• Type: RRAH
• Computes address from base register and absolute offset

Opcode	Mnemonic	Operation
0x4D	LD	#0 ← $3[#1 + $2]
0x4E	ST	$3[#1 + $2] ← #0

Relative addressing

• Type: RROH
• Computes address from register and offset from program counter

Opcode	Mnemonic	Operation
0x4F	LDR	#0 ← $3[pc + #1 + $2]
0x50	STR	$3[pc + #1 + $2] ← #0

Block memory copy

• Type: RRH
• Copies block of $3 bytes from memory location on address on #0 to #1

Opcode	Mnemonic	Operation
0x51	BMC	$3[#1] ← $3[x0]

Block register copy

• Type: RRB
• Copy block of $3 registers starting with #0 to #1
• Copying over the 256 registers causes an exception

Opcode	Mnemonic	Operation
0x52	BRC	$3#1 ← $3#0

Relative jump

• Type: O

Opcode	Mnemonic	Operation
0x53	JMP	pc ← pc + $0

Linking jump

• Operation:
  • Save address of following instruction to #0
    • #0 ← pc+<instruction size>
  • Jump to specified address

Opcode	Mnemonic	Instruction type	Address
0x54	JAL	RRO (size = 6 B)	Relative, pc + #1 + $2
0x55	JALA	RRA (size = 10 B)	Absolute, #1 + $2

Conditional jump

• Perform comparsion, if operation met, jump to relative address
• Type: RRP
• Operation: if #0 <CMP> #1 { pc ← pc + $2 }

Opcode	Mnemonic	Condition	Type
0x56	JEQ	Equals (=)	Xi64
0x57	JNE	Not-equals (≠)	Xi64
0x58	JLTU	Less-than (<)	Ui64
0x59	JGTU	Greater-than (>)	Ui64
0x5A	JLTS	Less-than (<)	Si64
0x5B	JGTS	Greater-than (>)	Si64

Environment traps

• Traps to the environment
• Type: N

Opcode	Mnemonic	Trap type
0x5C	ECA	Environment call
0x5D	EBP	Breakpoint

Floating point binary operations

• Type: RRR
• Operation: #0 ← #1 <OP> #2

Opcode	Mnemonic	Operation	Type
0x5E	FADD32	Addition (+)	Fl32
0x5F	FADD64	Addition (+)	Fl64
0x60	FSUB32	Subtraction (-)	Fl32
0x61	FSUB64	Subtraction (-)	Fl64
0x62	FMUL32	Multiplication (*)	Fl32
0x63	FMUL64	Multiplication (*)	Fl64
0x64	FDIV32	Division (/)	Fl32
0x65	FDIV64	Division (/)	Fl64

Fused multiply-add

• Type: RRRR
• Operation: #0 ← (#1 * #2) + #3

Opcode	Mnemonic	Type
0x66	FMA32	Fl32
0x67	FMA64	Fl64

Comparsions

• Type: RRR
• Operation: #0 ← #1 <=> #2
• Comparsion table same as for CMPx/CMPxI
• NaN is less-than/greater-than depends on variant

Opcode	Mnemonic	Type	NaN is
0x6A	FCMPLT32	Fl32	<
0x6B	FCMPLT64	Fl64	<
0x6C	FCMPGT32	Fl32	>
0x6D	FCMPGT64	Fl64	>

Int to float

• Type: RR
• Converts from Si64
• Operation: #0 ← Fl<SIZE>(#1)

Opcode	Mnemonic	Type
0x6E	ITF32	Fl32
0x6F	ITF64	Fl64

Float to int

• Type: RRB
• Operation: #0 ← Si64(#1)
• Immediate $2 specifies rounding mode

Opcode	Mnemonic	Type
0x70	FTI32	Fl32
0x71	FTI64	Fl64

Fl32 to Fl64

• Type: RR
• Operation: #0 ← Fl64(#1)

Opcode	Mnemonic
0x72	FC32T64

Fl64 to Fl32

• Type: RRB
• Operation: #0 ← Fl32(#1)
• Immediate $2 specified rounding mode

Opcode	Mnemonic
0x73	FC64T32

16-bit relative address instruction variants

Opcode	Mnemonic	Type	Variant of
0x74	LRA16	RRP	LRA
0x75	LDR16	RRPH	LDR
0x76	STR16	RRPH	STR
0x77	JMP16	P	JMP

psABI

C datatypes and alignment

• One byte is 8 bits

C Type	Description	Byte sizes
char	Character / byte	1
short	Short integer	2
int	Integer	4
long	Long integer	8
long long	Long long integer	8
float	Single-precision float	4
double	Double-precision float	8
long double	Extended-precision float	8

• Bikeshedding note: long double is now 8 bytes as the base ISA does not support f128. an extension for that should be made.

Call convention

• Registers r1 – r31 are caller saved
• Registers r32 – r255 are callee saved

Register	Description	Saver
r0	Hard-wired zero	N/A
r1 - r2	Return values	Caller
r2 - r11	Function parameters	Caller
r12 - r30	General purpose	Caller
r31	Return address	Caller
r32 - r253	General purpose	Callee
r254	Stack pointer	Callee
r255	Thread pointer	N/A

• If return value is too big to fit r1, r2 is also used.
• Values larger than two double-words are passed by reference