Machine Language Instruction Sets
A Typical Instruction Set
- Data Transfer
- Load, store, push, pop.
- Arithmetic
- Add, subtract, multiply, divide.
- Logical
- AND, OR, NOT, shift rotate.
- Test and Compare
- Control Flow
- Conditional jump, unconditional jump, subroutine call and return.
- Other
All high-level language code must be translated into these basic machine code instructions.
The difference between machine code and assembly are that machine code are the bytes that the CPU reads. Assembly is 1 to 1 with machine code but gives the instructions names and allows you to give memory addresses names.
Instruction Format
Opcode
Specifies the operation to be performed.
Operand Specifiers
The locations of source operands and destinations for results.
Often one of the operand will act as both source and destination.
Addressing Modes
The locations of operands are specified using addressing modes:
- Immediate
- The operand value is encoded directly into the instruction.
- Register
- The operand resides in a CPU register.
- Direct
- The operand resides in main memory; the instruction encodes its address.
- Register Indirect
- The instruction specifies a register, which in turn holds the address of the operand.
Generating Instructions
The machine code for high level languages is made by a compiler.
HLL Translation Process
graph LR
Editor --> SP[Source Program]
SP --> Compiler
Compiler --> OC[Object Code]
OC --> Linker
LM[Library Modules] --> Linker
Linker --> EP[Executable Program]
The compiler analyses the source code, checks for certain types of error, and generate equivalent object code.
The user’s code is integrated with pre-compiled library routines by a linker to form final the executable.
A single HLL statement may convert to many low-level instructions.
This process allows making human readable code that can run on the CPU.