Opcode Encoding - philpax/skiron GitHub Wiki

Encoding A

Used for three-register instructions.

Field Layout

6 bits	2 bits	2 bits	6 bits	6 bits	6 bits	2 bits	2 bits
opcode	encoding	variant	register1	register2	register3	padding	operandSize

• opcode (ubyte, 6 bits): The opcode number.
• encoding (Encoding, 2 bits): The encoding in use.
• variant (Variant, 2 bits): The variant/modifier to apply to register3.
• register1 (Register, 6 bits): The destination register.
• register2 (Register, 6 bits): The first source register.
• register3 (Register, 6 bits): The second source register.
• padding (ubyte, 2 bits)
• operandSize (OperandSize, 2 bits): The sizes of the operands being used.

Encoding B

Used for one-register, one-immediate instructions.

Field Layout

6 bits	2 bits	2 bits	6 bits	16 bits
opcode	encoding	variant	register1	immediate

• opcode (ubyte, 6 bits): The opcode number.
• encoding (Encoding, 2 bits): The encoding in use.
• variant (Variant, 2 bits): The variant/modifier to apply to the immediate.
• register1 (Register, 6 bits): The destination/source register.
• immediate (ushort, 16 bits): The encoded unsigned immediate value.

Encoding C

Used for one-immediate instructions.

Field Layout

6 bits	2 bits	2 bits	20 bits	2 bits
opcode	encoding	variant	immediate	operandSize

• opcode (ubyte, 6 bits): The opcode number.
• encoding (Encoding, 2 bits): The encoding in use.
• variant (Variant, 2 bits): The variant/modifier to apply to the immediate.
• immediate (int, 20 bits): The encoded signed immediate value.
• operandSize (OperandSize, 2 bits): The sizes of the operands being used.

Encoding D

Used for two-register, one-immediate instructions.

Field Layout

6 bits	2 bits	2 bits	6 bits	6 bits	8 bits	2 bits
opcode	encoding	variant	register1	register2	immediate	operandSize

• opcode (ubyte, 6 bits): The opcode number.
• encoding (Encoding, 2 bits): The encoding in use.
• variant (Variant, 2 bits): The variant/modifier to apply to the immediate.
• register1 (Register, 6 bits): The destination register.
• register2 (Register, 6 bits): The source register.
• immediate (int, 8 bits): The encoded signed immediate value.
• operandSize (OperandSize, 2 bits): The sizes of the operands being used.

Variants

The last argument of an instruction can be modified by a Variant before being used in a computation.

• Identity
  • Index: 0
  • Description: Pass the operand through unchanged.
• ShiftLeft1
  • Index: 1
  • Description: Shift the operand 1 bit to the left.
• ShiftLeft2
  • Index: 2
  • Description: Shift the operand 2 bits to the left.

Operand Formats

An opcode has an operand format, which specifies which arguments it takes.

• Dst
  • Description: Destination (register)
  • Encoding: A
  • Operand size support: true
• Uimm
  • Description: Source (unsigned immediate)
  • Encoding: B
  • Operand size support: false
• DstSrc
  • Description: Destination (register), source (register)
  • Encoding: A
  • Operand size support: true
• DstSrcSrc
  • Description: Destination (register), source (register), source (register)
  • Encoding: A
  • Operand size support: true
• DstUimm
  • Description: Destination (register), source (unsigned immediate)
  • Encoding: B
  • Operand size support: false
• DstSrcImm
  • Description: Destination (register), source1 (register), source2 (immediate)
  • Encoding: D
  • Operand size support: true
• Label
  • Description: Label (immediate)
  • Encoding: C
  • Operand size support: false
• None
  • Description: No operands
  • Encoding: A
  • Operand size support: false