Memory operations & instructions

Memory operations

to execute an instruction, processor control circuits have to cause word/words containing instruction to be transferred from memory to processor (with operands and results)

basic operations

• read — transfers copy of contents of memory location to processor
• write — transfers item of info from processor to a memory location

RISC vs CISC instruction sets

(most processors have a compromise)

RISC (Reduced Instruction Set Computers)

• simple addressing modes
• each instruction fits in one word
• fewer instructions in set
• arithmetic/logic only on operands in processor registers
• load/store architecture: no direct transfer between memory locations, must be through processor register
• programs tend to be larger in size (more, simpler instructions)

CISC (Complex Instruction Set Computers)

• more complex addressing modes, e.g.:
  • autoincrement— access operand through effective address in passed register, then increment contents of said register
  • autodecrement — contents of passed register are decremented and then used as effective address of operand
  • relative — effective address is determined using index mode with PC instead of register
• instructions don’t have to fit into a single word
• more complex instructions
• arithmetic/logic also on both memory locations and registers
• not constrained to load/store architecture

Instruction Execution

the processor has a program counter (PC) register, holds address of next instruction

processor circuits use info in PC to fetch and execute instructions in order of increasing address (straight-line sequencing)

executing an instruction

1. Instruction fetch — instruction fetched from mem location stored in PC and put in instruction register

2. Instruction execute — instruction is examined and the operation is performed, PC is incremented by 4.

branch instructions load new address into PC, allow conditional jumping (jump if greater/less than)