Q.What is an operational code (opcode)?

The 8051 instruction set contains 111 instructions, and every one is written as a mnemonic (the operational code, or opcode) followed by its operands. The opcode is the "verb" that says what to do; the operands are the data it works on. The instructions fall into four groups.

1. Data-transfer instructions move data without changing it — MOV (within internal memory), MOVX (to/from external RAM), MOVC (from code memory), the stack instructions PUSH and POP, and the exchange instructions XCH/XCHD. PUSH first increments the stack pointer then stores a byte; POP retrieves a byte then decrements the pointer. These generally leave the flags unchanged.

2. Arithmetic instructions operate on 8-bit data through the accumulator: ADD, ADDC, SUBB, INC, DEC, MUL AB, DIV AB and DA A.

3. Logical instructions perform bitwise operations — ANL (AND), ORL (OR), XRL (Exclusive-OR) — plus CLR, SETB, CPL, the rotates RL/RR/RLC/RRC and SWAP. The bitwise logic operations affect no flags, while RLC and RRC update the carry flag. …