lecture3.pptx

Memory Address Space

We can directly memory map peripherals into the address space (e.g. a mouse).

Ex: LD r0 0xFFFE

(if the mouse is at the bus on 0xFFFE).

What’s included in an ISA?

• Registers
• Instructions
  • Privileged vs Unprivileged
• Instruction format
• Data types
  • 32, 64-bit
  • Integers, floating-point
• Addressing modes
  • Immediate
    • Operand directly embedded in the instruction
  • Register
  • Direct
    • Memory operand directly embedded within the instruction
    • LD r0, #5
    • Cons: memory latency, limited range (due to instruction size), no dynamic offsets
  • Indirect
    • Specifies register that contains address of operand
    • ADD r0, (r1)
    • Cons: indirection (complex execution)
  • Displacement
    • Add index to offset
    • ADD r0, #5(r1)
  • Indexed
    • Add index to offset but from another register
    • ADD r0, r1(r2)
  • Relative
    • Operand is a memory address relative to program counter
    • JMP +4
  • Stack
    • Operands implicitly located on some stack
    • PUSH r0
    • POP r0
• Memory alignment
  • Restrict to accesses that are multiples of some fixed value (or else would need to access multiple)
• Endianness
• Control/status regs
• Extensions

Example: 6502 8-bit CPU

Popular in 80’s. Variable length instructions 1-3 bytes. 1 byte opcode. 1-2 bytes operands.

5 registers: A, X, Y, SP, SR.

Accumulator: used for immediate operations. LDA 0x10.