CS 311 Exam Terms (TRADEOFFS)

Chapters 4,5,6,7,8

Tradeoff Sentence: Harvard vs Von Neumann

Harvard trades flexibility for the ability to optimize each memory; Von Neumann trades some of that optimization for flexibility, which is why most computers use it.

Tradeoff sentence: fixed vs. variable length

Fixed-length wastes some memory but is much faster to fetch and decode (hardware is iterated in space);

Variable-length saves memory but needs slower, iterative decoding

Tradeoff sentence: NUMBER of OPERANDS

More operands (3-Address) means fewer instructions and easier programming but longer instructions; fewer operands means smaller, simpler instructions but more of them. No perfect solution, only tradeoffs.

Tradeoff sentence: CISC vs RISC

CISC gives powerful, compact programs but complex hardware and slower instructions; RISC uses a few simple fixed-sized instructions that can complete one per cycle but needs more instructions per program.

Tradeoff sentence: SRAM vs DRAM

SRAM is fast but uses more power and space; DRAM is slower, needs refresh, but uses less power and fits more bits, so it is main memory.

Tradeoff sentence: Registers vs Memory

Registers are fast but few and expensive; memory is large and cheap but slow. Keep values you will reuse in registers.

Tradeoff sentence: Immediate Values

Fast and no register or memory needed, but the value must be known ahead of time and is limited by the bits available in the instruction.

Tradeoff Sentence: Passing arguments in registers vs on the stack!

Registers are faster but compete with computation for registers; the stack is unlimited but in slow memory. ARM passes the first three arguments in registers.

Design Tradeoff (GENERAL)

A balance among competing goals (hardware cost, speed, power, programmer convenience, instruction size). Improving one usually hurts another.