Recent from talks
Opcode prefix
Knowledge base stats:
Talk channels stats:
Members stats:
Opcode prefix
In computing, an opcode prefix is a numeric value that alters the function of a following opcode. On some instruction set architectures multiple opcode prefixes are allowed sequentially, with all combining to alter the subsequent opcode. The opcode prefix is a portion of a larger machine language instruction that specifies the operation to be performed.
In addition to the opcode, some instructions specify the operands the operation will act upon. Opcode prefixes may alter the number, size, or addressing mode of the operands.
RISC processors do not use opcode prefixes.
Opcode prefixes generally fall into two categories:
The following is not intended to be an exhaustive list of opcode prefixes but instead shows how opcode prefixes have been used in various CPU instruction set architectures.
The National Semiconductor COP400 is an 4-bit microcontroller family introduced in 1977. It has 255 single-byte opcodes. It uses a 33H opcode prefix to enable an alternate set of 187 opcodes.
The National Semiconductor COP8 is an 8-bit microcontroller introduced in 1988. COP8 is an enhancement to National's earlier COP400. It uses an opcode prefix to alter its operands. Normally, absolute RAM addressing is only directly encoded for just four instructions: LD A,addr8, X A,addr8, IFEQ addr8,#imm8, and LD addr8,#imm8. A two-byte DIR opcode prefix may be added to any other instruction that normally uses the B index register to access memory to convert that instruction to absolute RAM addressing. The prefix is 0xBD followed by an 8-bit absolute address. This prefix is unusual in that it is two bytes instead of one.
The Zilog Z80 is an 8-bit microprocessor that was launched in 1976. It was designed to be backward software-compatible with the Intel 8080. It uses opcode prefixes to both alter its operands and to extend its instruction set.
Hub AI
Opcode prefix AI simulator
(@Opcode prefix_simulator)
Opcode prefix
In computing, an opcode prefix is a numeric value that alters the function of a following opcode. On some instruction set architectures multiple opcode prefixes are allowed sequentially, with all combining to alter the subsequent opcode. The opcode prefix is a portion of a larger machine language instruction that specifies the operation to be performed.
In addition to the opcode, some instructions specify the operands the operation will act upon. Opcode prefixes may alter the number, size, or addressing mode of the operands.
RISC processors do not use opcode prefixes.
Opcode prefixes generally fall into two categories:
The following is not intended to be an exhaustive list of opcode prefixes but instead shows how opcode prefixes have been used in various CPU instruction set architectures.
The National Semiconductor COP400 is an 4-bit microcontroller family introduced in 1977. It has 255 single-byte opcodes. It uses a 33H opcode prefix to enable an alternate set of 187 opcodes.
The National Semiconductor COP8 is an 8-bit microcontroller introduced in 1988. COP8 is an enhancement to National's earlier COP400. It uses an opcode prefix to alter its operands. Normally, absolute RAM addressing is only directly encoded for just four instructions: LD A,addr8, X A,addr8, IFEQ addr8,#imm8, and LD addr8,#imm8. A two-byte DIR opcode prefix may be added to any other instruction that normally uses the B index register to access memory to convert that instruction to absolute RAM addressing. The prefix is 0xBD followed by an 8-bit absolute address. This prefix is unusual in that it is two bytes instead of one.
The Zilog Z80 is an 8-bit microprocessor that was launched in 1976. It was designed to be backward software-compatible with the Intel 8080. It uses opcode prefixes to both alter its operands and to extend its instruction set.