Arrays

array[index]

• This is done by using the address register

• How to implement
  1. Load base address (start of array) into address register
  2. Load index i into accumilator
  3. Add address register to accumulator, send result to address register
  4. Use address register to access memory

• Repeated Sequential access - after starting with the sequence use the auto-increment facility

Functions

Stored Program Concept

• Address of functions first instruction must be known

• Return address must be stored, so that can resume

• Note specialised register is useless if functions are nested, solved with a stack

Stack

• Stacks have a specialised register known as a stack pointer (SP)

PUSH

• Decrement SP, (grow stack) store value where SP is pointing

POP

• retrieve value then increment SP (note value of the stack will be overwritten)

SubRoutine

Jump to SubRoutine (JSR)

• Performs the jump to the first instruction

• Pushes return address from program counter onto stack

Return to SubRoutine(RTS)

• Pops stored return address back into program counter from

Stack Pointer wiring

The layout of the address register is the same as the stack pointer except it has a adder

Different addressing modes

Directives - used to define variables, giving them a : type, name, space in memory, value

Identifiers - this is the chosen name for a variable

• Assembly language uses identifiers just like high level code

• Identifiers are declared by a directive that allocates memory and assosiates the address with an identifier

• We must assume functionality of a instruction is encoded with the opcode (makes life hard)

• Real world assembly languages assessing operands is encoded using addressing modes

Immediate

#4 - # followed by address

Load the address

Direct

4 - just the address

Operand is the address of the value to be used

Indirect

(A0)

-(A0) - pre-decriment first

(A0)+ - increment first

Operand is the address of the address of the data to be used

Indexed (Displacement)

Operand + Index register = address of data to be loaded

Relative

Operand + Address register = address of data to be loaded