How long is an x86 instruction?

between 1 and 15 bytes
x86 instructions can be anywhere between 1 and 15 bytes long. The length is defined separately for each instruction, depending on the available modes of operation of the instruction, the number of required operands and more.

How many instructions are in x86?

According to Intel’s XED, as of this writing, there are 1503 defined x86 instructions (“iclasses” in XED lingo), from AAA to XTEST (this includes AMD-specific extensions too, by the way).

What size memory does the x86 DB instruction allocate?

Addressing Memory Modern x86-compatible processors are capable of addressing up to 232 bytes of memory: memory addresses are 32-bits wide. In the examples above, where we used labels to refer to memory regions, these labels are actually replaced by the assembler with 32-bit quantities that specify addresses in memory.

Are x86 instructions fixed length?

One of the advantages that x86 has over most RISC chips is instruction density. x86 instructions are variable-length, which means that common instructions typically have a shorter encoding and so take up less space in instruction cache. Therefore, x86 chips need smaller instruction caches for the same performance.

How do you subtract arms?

Arithmetic Instructions

  1. ADD – Add. Rd := Rn + Operand2.
  2. ADC – Add with Carry. Rd := Rn + Operand2 + Carry.
  3. SUB – Subtract. Rd := Rn − Operand2.
  4. SBC – Subtract with Carry. Rd := Rn − Operand2 − NOT(Carry)
  5. RSB – Reverse Subtract. Rd := Operand2 − Rn.
  6. RSC – Reverse Subtract with Carry. Rd := Operand2 − Rn − NOT(Carry)

How many operands does DEC instruction have?

The dec instruction decrements the contents of its operand by one. dec eax — subtract one from the contents of EAX. The imul instruction has two basic formats: two-operand (first two syntax listings above) and three-operand (last two syntax listings above).

What are the bit and byte instructions in x86?

Bit and Byte Instructions (x86 Assembly Language Reference Manual) Documentation Home > x86 Assembly Language Reference Manual > Chapter 3 Instruction Set Mapping > General-Purpose Instructions > Bit and Byte Instructions x86 Assembly Language Reference Manual Previous: Shift and Rotate Instructions Next: Control Transfer Instructions

What is the opcode size of the x86?

The x86 CPU supports two basic opcode sizes: two -byte opcode consisting of a 0Fh opcode expansion prefix byte . The second byte then specifies the actual instruction. The x86 opcode bytes are 8-bit equivalents of iii field that we discussed in simplified encoding.

Are there any instructions that fit into one byte?

Some instructions may have an opcode and operands… but they will still fit into one byte. These typically encode a register value into a part of the byte. After building this list, I started a new list to include instructions that are a total of two bytes. These may be two byte opcodes or an opcode and a single byte operand.

What are the different types of x86 instructions?

Machine instructions generally fall into three categories: data movement, arithmetic/logic, and control-flow. In this section, we will look at important examples of x86 instructions from each category. This section should not be considered an exhaustive list of x86 instructions, but rather a useful subset.