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Flashcards · 9618 · AS Topic 4

Processor Fundamentals (AS Level) flashcards

17 cards on AS Topic 4, subtopics 4.1 to 4.3. Pick a subtopic or study them all.

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4.1 Central Processing Unit (CPU) Architecture

What is the Von Neumann model of a computer?
A single processor that fetches and executes instructions in sequence, with program instructions and data stored together in the same main memory (the stored program concept).
What do the PC, MAR, MDR and CIR hold?
PC: address of the next instruction. MAR: address of the location currently being accessed. MDR: data or instruction being transferred to or from memory. CIR: the instruction currently being decoded and executed.
What do the accumulator (ACC), index register (IX) and status register hold?
ACC: the result of ALU operations. IX: a value added to an address in indexed addressing. Status register: flag bits (carry, negative, overflow, zero) set by the last operation.
What are the steps of the fetch stage in register transfer notation?
MAR ← [PC] PC ← [PC] + 1, and MDR ← [[MAR]] CIR ← [MDR]
What do the address, data and control buses carry?
Address bus: addresses from the MAR to memory (one way). Data bus: data and instructions (both ways). Control bus: control signals such as memory read, memory write and the clock (both ways).
Which factors affect the performance of a CPU?
Clock speed, number of cores, size of cache memory, and the width of the address and data buses.
What is an interrupt and how is it handled?
A signal that a device or program needs attention. At the end of each fetch-execute cycle the CPU checks for interrupts; if one has priority, the current register contents are saved, the interrupt service routine runs, then the saved values are restored.

4.2 Assembly Language

What is the relationship between assembly language and machine code?
Each assembly instruction is a mnemonic for one machine-code instruction. An assembler translates assembly language into machine code.
What is the difference between immediate and direct addressing?
Immediate: the operand is the value itself (LDM #5 loads 5). Direct: the operand is the address of the value (LDD 200 loads the contents of address 200).
What is the difference between indirect and indexed addressing?
Indirect: the operand holds the address of the address of the value (LDI). Indexed: the address used is the operand plus the contents of the index register (LDX).
What is relative addressing?
The operand is an offset; the address used is worked out relative to a base address, usually that of the current instruction.
How does a conditional jump such as JPE work after CMP?
CMP compares the ACC with the operand. JPE jumps to the given address only if the comparison was equal; otherwise execution continues with the next instruction.
What is a two-pass assembler?
Pass 1 reads the program and builds a symbol table of labels and their addresses. Pass 2 uses the table to translate every instruction, including forward references to labels, into machine code.

4.3 Bit manipulation

How do you clear, set or toggle chosen bits with a mask?
Clear: AND with 0s in those positions. Set: OR with 1s in those positions. Toggle: XOR with 1s in those positions.
What does a logical left shift do to an unsigned binary value?
Each bit moves left and 0 enters on the right. A shift of n places multiplies the value by 2ⁿ (bits shifted out are lost).
What is the difference between a logical and an arithmetic right shift?
Logical: 0 enters on the left. Arithmetic: the sign bit is copied in, so a negative two’s complement value stays negative. Either divides by 2 per place.
What is a cyclic shift?
Bits shifted out of one end are moved back in at the other end, so no bits are lost.

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