Machine Instructions are commands or programs written in machine code of a machine (computer) that it can recognize and execute.
- A machine instruction consists of several bytes in memory that tells the processor to perform one machine operation.
- The processor looks at machine instructions in main memory one after another, and performs one machine operation for each machine instruction.
- The collection of machine instructions in main memory is called a machine language program.
Machine code or machine language is a set of instructions executed directly by a computer’s central processing unit (CPU). Each instruction performs a very specific task, such as a load, a jump, or an ALU operation on a unit of data in a CPU register or memory. Every program directly executed by a CPU is made up of a series of such instructions.
The general format of a machine instruction is
| [Label:] Mnemonic [Operand, Operand] [;Comments] |
- Brackets indicate that a field is optional
- Label is an identifier that is assigned the address of the first byte of the instruction in which it appears. It must be followed by “:”
- Inclusion of spaces is arbitrary, except that at least one space must be inserted; no space would lead to an ambiguity.
- Comment field begins with a semicolon “ ; ”
Example:
| Pointing: MOV R5, #25H ;load 25H into R5 |
Machine instructions used in 8086 microprocessor
1. Data transfer instructions– move, load exchange, input, output.
- MOV :Move byte or word to register or memory .
- IN, OUT: Input byte or word from port, output word to port.
- LEA: Load effective address
- LDS, LES Load pointer using data segment, extra segment .
- PUSH, POP: Push word onto stack, pop word off stack.
- XCHG: Exchange byte or word.
- XLAT: Translate byte using look-up table.
2. Arithmetic instructions – add, subtract, increment, decrement, convert byte/word and compare.
- ADD, SUB: Add, subtract byte or word
- ADC, SBB :Add, subtract byte or word and carry (borrow).
- INC, DEC: Increment, decrement byte or word.
- NEG: Negate byte or word (two’s complement).
- CMP: Compare byte or word (subtract without storing).
- MUL, DIV: Multiply, divide byte or word (unsigned).
- IMUL, IDIV: Integer multiply, divide byte or word (signed)
- CBW, CWD: Convert byte to word, word to double word
- AAA, AAS, AAM,AAD: ASCII adjust for add, sub, mul, div .
- DAA, DAS: Decimal adjust for addition, subtraction (BCD numbers)
3. Logic instructions – AND, OR, exclusive OR, shift/rotate and test
- NOT : Logical NOT of byte or word (one’s complement)
- AND: Logical AND of byte or word
- OR: Logical OR of byte or word.
- XOR: Logical exclusive-OR of byte or word
- TEST: Test byte or word (AND without storing).
- Shift, rotate instruction- SHL, SHR Logical shift left, right byte or word? by 1or CL
- SAL, SAR Arithmetic shift left, right byte or word? by 1 or CL
- ROL, ROR Rotate left, right byte or word? by 1 or CL .
- RCL, RCR Rotate left, right through carry byte or word? by 1 or CL.
- String manipulation instruction – load, store, move, compare and scan for byte/word
- MOVS: Move byte or word string
- MOVSB, MOVSW: Move byte, word string.
- CMPS: Compare byte or word string.
- SCAS S: can byte or word string (comparing to A or AX)
- LODS, STOS: Load, store byte or word string to AL.
5. Control transfer instructions – conditional, unconditional, call subroutine and return from subroutine.
- JMP:Unconditional jump .it includes loop transfer and subroutine and interrupt instructions.
6. Loop control instructions-
- LOOP: Loop unconditional, count in CX, short jump to target address.
- LOOPE (LOOPZ): Loop if equal (zero), count in CX, short jump to target address.
- LOOPNE (LOOPNZ): Loop if not equal (not zero), count in CX, short jump to target address.
- JCXZ: Jump if CX equals zero (used to skip code in loop).
- Subroutine and Intrrupt instructions-
- CALL, RET: Call, return from procedure (inside or outside current segment).
- INT, INTO: Software interrupt, interrupt if overflow.IRET: Return from interrupt.
7. Processor control instructions-
Flag manipulation:
- STC, CLC, CMC: Set, clear, complement carry flag.
- STD, CLD: Set, clear direction flag.STI, CLI: Set, clear interrupt enable flag.
- PUSHF, POPF: Push flags onto stack, pop flags off stack.
