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Later revisions added opcode bits, allowing additional address bits. They are accumulator machines , with a common accumulator "W" being one operand in all 2-operand instructions. In the instruction set tables that follow, register numbers are referred to as "f", while constants are referred to as "k".
The opcode for INT3 is 0xCC, as opposed to the opcode for INT immediate8, which is 0xCD immediate8. Since the dedicated 0xCC opcode has some desired special properties for debugging, which are not shared by the normal two-byte opcode for an INT3, assemblers do not normally generate the generic 0xCD 0x03 opcode from mnemonics.
Undefined Instructions – will generate an invalid opcode (#UD) exception in all operating modes. [z] These instructions are provided for software testing to explicitly generate invalid opcodes. The opcodes for these instructions are reserved for this purpose. (3) , [aa] Intel Pentium [68] UD1 reg,r/m, [ab] UD2B reg,r/m [y] 0F B9 /r [ac] OIO, UD0,
An opcode table (also called an opcode matrix) is a visual representation of all opcodes in an instruction set. It is arranged such that each axis of the table represents an upper or lower nibble, which combined form the full byte of the opcode. Additional opcode tables can exist for additional instructions created using an opcode prefix.
Ralf Brown's Interrupt List (aka RBIL, x86 Interrupt List, MS-DOS Interrupt List or INTER) is a comprehensive list of interrupts, calls, hooks, interfaces, data structures, CMOS settings, memory and port addresses, as well as processor opcodes for x86 machines from the 1981 IBM PC up to 2000 (including many clones), [1] [2] [nb 1] most of it still applying to IBM PC compatibles today.
In computing, an opcode (abbreviated from operation code) [1] [2] is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs) and central processing units (CPUs) as well as in some software instruction sets.
The REX prefix provides additional space for encoding 64-bit addressing modes and additional registers present in the x86-64 architecture. Bit-field W changes the operand size to 64 bits, R expands reg to 4 bits, B expands r/m (or opreg in the few opcodes that encode the register in the 3 lowest opcode bits, such as "POP reg"), and X and B expand index and base in the SIB byte.
Four bits R̅, X̅, B̅ and W from the VEX prefix, stored in inverted form. W expands the operand size to 64 bits or serves as an additional opcode, R expands reg, B expands r/m or reg, and X and B expand index and base in the SIB byte. Four bits named v̅, stored in inverted form. vvvv specifies a second non-destructive source register operand.