Stretch tasks Systems Architecture

Round 1: Quiz Blitz

1. Instructions and data are held in the same memory That is why one laptop can run any program without rewiring.
2. Instructions and data compete for the same bus to memory The processor can spend much of its time waiting.
3. Program Counter Its value is copied into the MAR at the start of the fetch.
4. False It is incremented during the fetch, so it already points at the next instruction.
5. The address in the PC is copied into the MAR → The PC is incremented → The address goes out on the address bus → The instruction comes back on the data bus into the MDR PC to MAR, increment, address bus out, data bus back.
6. The control unit The control unit works out what operation is needed and what data it applies to.
7. Power use and heat rise faster than the speed Beyond a point, the chip cannot be cooled.
8. Not all software splits its work across cores; Some tasks need each step's result before the next can start Some work is inherently sequential, and much software is not written for many cores.
9. False It reduces the time the processor spends waiting, so more instructions are completed per second.
10. It is expensive per byte Cache is very fast, and very costly.
11. A microwave's controller It is built into a larger device to do one dedicated task.
12. accumulator The ALU puts its result in the accumulator.

Round 2: Quick Fire

1. • So it already points at the next instruction
   • A jump writes a new address into the PC, which a later increment would spoil
   Model answer

   Incrementing during the fetch means the PC already points at the next instruction. If it waited until the end, a jump executed in this cycle would write a new address into the PC, only for the increment to overwrite it.

2. • It holds recently and frequently used instructions and data close to the CPU
   • So the CPU spends less time waiting for RAM
   Model answer

   Cache holds recently and frequently used instructions and data inside or very close to the CPU, so the processor spends less time waiting for slow RAM and completes more instructions per second.

3. • Its task is fixed, so it needs only a small processor, little memory and no full OS
   • With fewer, simpler parts, there is less to go wrong
   Model answer

   Its task is fixed, so it needs only a small processor, very little memory and no general purpose operating system. With fewer, simpler parts there is less to pay for and less to go wrong.

4. • The MAR holds the address of the instruction being fetched
   • The MDR holds the instruction that comes back from memory
   Model answer

   The MAR holds the address of the instruction being fetched. The MDR holds the instruction that comes back from memory at that address.

Round 3: Big Think

1. • The address in the PC is copied into the MAR
   • The PC is incremented
   • The address goes out on the address bus
   • The instruction comes back on the data bus into the MDR
   • The control unit decodes it
   • The ALU executes a calculation, with the result in the accumulator
   • A jump writes a new address into the PC
   • Then the cycle repeats
   Model answer

   The address in the PC is copied into the MAR, and the PC is incremented so it points at the next instruction. The address travels out along the address bus, and the instruction at that address comes back along the data bus into the MDR. The control unit decodes the instruction, working out what operation is needed. Then it is executed: a calculation is carried out by the ALU with the result in the accumulator, or a jump writes a new address into the PC. Then the cycle repeats, billions of times a second.

2. • 3.6 GHz is 3.6 billion clock pulses per second
   • So more instructions are completed each second
   • But power use and heat rise faster than the speed
   • Four cores can run four instructions at the same instant
   • But only if the software splits the work, and some tasks are sequential
   • Cache cuts the time spent waiting for RAM
   • But cache is expensive per byte
   Model answer

   3.6 GHz means 3.6 billion clock pulses per second, so more instructions are completed each second, but power use and heat rise faster than the speed, which limits how high it can go. Four cores can execute four instructions at the same instant, but only if the software is written to split its work, and tasks where each step needs the previous result gain little. The 8 MB of cache keeps recently used data close to the CPU, so it spends less time waiting for RAM, but cache is expensive per byte, which is why there is not more of it.

This or That

1. MAR The Address Register holds an address. The Data Register holds data.
2. By rewiring it The stored program concept ended physical rewiring.
3. The accumulator The PC holds the address of the next instruction.
4. Rise even faster Heat and power climb faster than speed, which is why clock speeds stalled.
5. Very little Sequential work cannot be split across cores.
6. It is inside or very close to the CPU Fetching from cache takes a fraction of the time of fetching from RAM.
7. A washing machine controller An embedded system is dedicated to one task.
8. A new address goes into the PC The next fetch then starts from the new address.

Memory Match

• A 6 PC: Address of the next instruction
• B 3 MAR: Address being read or written
• C 4 MDR: Data just fetched, or about to be written
• D 2 ACC: Result of the latest calculation
• E 1 Stored program concept: Instructions and data share memory
• F 5 Von Neumann bottleneck: Instructions and data compete for one bus

Sort It

1. Cores No help for tasks where each step needs the last Sequential work cannot be shared out.
2. Cache Holds recently used instructions close to the CPU That is cache's job.
3. Clock speed Measured in hertz Hertz means per second.
4. Cache Cuts the time spent waiting for RAM Cache reduces waiting rather than speeding up the CPU.
5. Cores Each one is a complete processing unit That is what a core is.
6. Cores A quad core chip runs four instructions at once Each core works at the same instant.
7. Cores Only helps if software splits the work Software must be written for many cores.
8. Clock speed Heat and power rise faster than it does That is why clock speeds stopped climbing.
9. Clock speed One instruction stage happens per pulse The clock sets the pace.
10. Cache Expensive per byte, so there is little of it Cache is very fast and very costly.
11. Clock speed 3 GHz means three billion pulses a second Clock speed counts pulses per second.