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I need an explanation about the term "opcode". I have found this :

An opcode is binary or hex value of the machine instruction in the cpu instruction set.

If I understand correctly the definition, for instance :

6A 03 push 3

6A is the opcode et 03 the operand. But in several posts on different forums, people talk about 03 as if it was an opcode (for instance this

Here, 3 is an immediate because it's included directly as part of the opcode (in the second byte).

).

Does 03 is an opcode and an operand?

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    An instruction is composed of many parts which are not necessarily byte sized. Commented Dec 24, 2018 at 14:17
  • To reduce confusion, it's better to refer 6A 03 as the instruction with opcode and operand being 6A and 03 respectively. The mnemonic would be push 3. Commented Dec 24, 2018 at 14:48
  • Compare 0xCC INT 3 – an opcode and its operand, in a single byte. Commented Dec 24, 2018 at 22:01

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Regarding this specific push and according to the Intel SDM (Page can be identified by searching Vol. 2B 4-507) 03 is both a part of the opcode and the "argument". The full opcode of "push 03" is "6A 03"

As far as I know Intel SDM consistently uses the term opcode as "all the content of the assembly command, including operands". Table of possible PUSH opcodes in the SDM

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