- Processor Instructions
- Processors can only operate using switches (transistors)
- We give a processor instructions using binary
- Which correlates to switches being on or off
- We group switches into logic gates, and then logic gates into
useful circuits
- E.g. adding two numbers together
- Logic gate circuits enable everything in modern day computing
- We can group processor instructions into logical groups:
- Data movement
- Register to main memory and vice versa
- Input and output of data
- Arithmetic operations
- Unconditional and conditional jump instructions
- Compare instructions
- Modes of addressing
- There is a separate part of the CPU that manages calculations
and logic; called the ALU
- This handles much of the processor’s work
- Each CPU family will have its own instruction set
- This means that compilers and interpreters translating into
binary must be written for specific operating systems
and instruction sets
- Because binary is tough for humans, we invented assembly
language
- This uses mnemonics (codes) to substitute for the equivalent
binary instruction
- Which makes it easier for humans and less prone to errors
- Label
- References a memory location
- e.g. Start, End
- On first pass the label is converted back to its memory location
- Addressing modes
- There are various ways to address memory
- Immediate
- This value is the number to be used
- There is no link to another memory address to get the
required value
- Direct
- The value represents the memory location where the
required value can be found
- Indirect
- This value points to the memory address, which itself
contains an address where the value can be sought
- Indexed
- This value points to an address, which must also be
added to the value in the index register (IX) to
retrieve the value required
- Commonly used to reference arrays
- Relative
- This value represents an offset to the current memory
address in the program counter register
- E.g. if the PC was pointing to address 70, a
relative address of 5 would retrieve from address 75
- Commonly used when loading programs into memory by a
linker
- When writing a program there is no way of knowing where
in memory your program will sit
- Therefore, everything works by an offset
- Typical instructions (provided in an exam)
- The LMC Simulator
- The Little Man Computer (LMC) is a simulator that has many of
the basic features found in a Von Neumann architecture machine
- Such as memory being used for instructions and data
- Its premise is that the CPU is like a little man, fetching
instructions, controlling data in and out, etc.
- LMC uses its own instruction set and compiles the instructions,
uniquely, into decimal notation for easy reference
- It is an abstract representation so the real thing does
differ (for example how values are input and output)
- The LMC tasks will help you understand how assembly language is
sequenced