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Structure of the IAS Computer

The “sketch the IAS computer with its components” question (Summer-24 Q1c). Learn the block diagram and the seven registers.

The von Neumann model

All of today’s computers share the same general structure and are therefore called von Neumann machines. The IAS computer (Institute for Advanced Study, Princeton, ~1946–52) is the classic example, built by John von Neumann.

Four top-level components, with data flowing both ways between memory and the CPU:

Central Processing Unit (CPU)Main Memory(M)Arithmetic-LogicUnit (CA)Program ControlUnit (CC)I/O(I, O)
Top-level IAS structure — data flows both ways between memory, the CPU and I/O.
ComponentRole
Main Memory (M)Stores both data and instructions (stored-program concept).
Arithmetic-Logic Unit (CA)Performs the arithmetic and logic on data.
Program Control Unit (CC)Interprets instructions and sequences their execution.
I/O Equipment (I, O)Moves data in and out, directed by the control unit.
The ALU (CA) + Control Unit (CC) together make the CPU. Memory holds instructions and data — the defining idea of the stored-program (von Neumann) machine.

Full IAS block diagram — the registers

The detailed IAS diagram (Figure 1.6) adds the registers inside the CPU. These are frequently asked by name:

CPUArithmetic-Logic Unit (CA)ACMQAC = AccumulatorMQ = Multiply-QuotientMBR = Memory Buffer RegisterArithmetic-logic circuitsMBRProgram Control Unit (CC)PCIBRMARIRPC = Program CounterIBR = Instruction Buffer Reg.MAR = Memory Address RegisterIR = Instruction RegisterControl circuits → control signals
The seven IAS registers inside the CPU (Figure 1.6).
RegisterNamePurpose
ACAccumulatorHolds an operand / result of the ALU.
MQMultiply-QuotientHolds the other half of a multiply/divide result.
MBRMemory Buffer RegisterWord just read from / about to be written to memory.
IBRInstruction Buffer RegisterHolds the right-hand instruction of a word.
PCProgram CounterAddress of the next instruction pair to fetch.
MARMemory Address RegisterAddress of the memory word to be accessed.
IRInstruction RegisterOpcode of the instruction currently executing.

Memory hook: “A MoM Picked My IR” → AC, MQ, MBR, PC, MAR, IBR, IR.

Because arithmetic centres on the single AC, IAS is an accumulator machine: you LOAD a value into AC, operate on it, then STOR it back — the pattern behind every programming question.