holey-bytes/spec.md

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 n
• imm #n: for immediate in parameter n
• P ← V: Set register P to value V
• [x]: Address x

Program execution control

• N type

Opcode	Name	Action
0	UN	Trigger unreachable code trap
1	TX	Terminate execution
2	NOP	Do nothing

Integer binary ops.

• BBB type
• #0 ← #1 <op> #2

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
<	-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
14	DIR	Divide and remainder combinated

Negations

• Type BB
• #0 ← #1 <op> #2

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

• Type BB
• #0 ← #1

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

• Type BD
• #0 ← #1

Opcode	Name	Action
29	LI	Load immediate

Memory operations

• Type BBDH
• If loaded/store value exceeds one register size, continue accessing following registers

Load / Store

Opcode	Name	Action
30	LD	#0 ← [#1 + imm #3], copy imm #4 bytes
31	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
32	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
33	BRC	#1 ← #0, copy imm #2 registers

Control flow

Unconditional jump

• Type BBD

Opcode	Name	Action
34	JAL	Save PC past JAL to #0 and jump at #1 + imm #2

Conditional jumps

• Type BBD
• Jump at imm #2 if #0 <op> #1

Opcode	Name	Comparsion
35	JEQ	=
36	JNE	≠
37	JLT	< (signed)
38	JGT	> (signed)
39	JLTU	< (unsigned)
40	JGTU	> (unsigned)

Environment call

• Type N

Opcode	Name	Action
41	ECALL	Cause an trap to the host environment

Floating point operations

• Type BBB
• #0 ← #1 <op> #2

Opcode	Name	Action
42	ADDF	Addition
43	SUBF	Subtraction
44	MULF	Multiplication

Division-remainder

• Type BBBB

Opcode	Name	Action
45	DIRF	Same as for integer DIR

Fused Multiply-Add

• Type BBBB

Opcode	Name	Action
46	FMAF	#0 ← (#1 * #2) + #3

Negation

• Type BB

  Opcode	Name	Action
  47	NEGF	#0 ← -#1

Conversion

• Type BB
• Signed
• #0 ← #1 as _

Opcode	Name	Action
48	ITF	Int to Float
49	FTI	Float to Int

Floating point immediate operations

• Type BBD
• #0 ← #1 <op> imm #2

Opcode	Name	Action
50	ADDFI	Addition
51	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.