## Basic Registers on Bus Architecture

An example of the registers on bus architecture is shown below.

The basic registers on bus architecture computer is almost a finite state machine. The clock that controls the buffers, latches (on the bus) and memory is not shown. It is not quite a regular finite state machine because data can move from latch 1 to the memory in the block of logic comprised of the processor and memory.

The block of logic in the diagram above is labeled 'processor and memory.' However, the registers are not in the processor, though the logic unit (rotate and mask logic) is. Latch 1 is comprised of all the registers. In the processor in the book, there are 8 registers. If it is a 32-bit processor, there are 32 bits in each register and so 8 X 32 = 256 bits in all the registers. Therefore, there are 256 bits in latch 1. Of course, there are also 256 bits in latch 4.

In the computer in the book 'How Computers Work,' an instruction is done in nine steps. However, with small changes to the computer, it could have been done in four steps, as explained earlier. In this computer, an instruction is done in four steps. The first step copies the four words of the instruction to be executed, from the memory to four registers (in latch 4). The next step copies the 'from data' from the memory to a register (in latch 4). The next step copies the 'to data' from the memory to a register (in latch 4). The last step copies the result of using the rotate and mask operation on the 'from data' and 'to data' in registers (in latch 1) to the memory.

During each pass through the block of logic (labeled 'processor and memory' in the diagram), a register (or four) or latch in memory is updated. Only 6 registers are used for this. Four registers hold the instruction, one register holds the 'from data,' and one register holds the 'to data.' There are two registers left. One of the registers holds a three bit counter that counts from 0 to 3 (four numbers). The number indicates which step of the instruction is to be done on the next pass through the block of logic (labeled 'processor and memory') in the diagram.

That's all there is to this basic registers on bus computer. It runs the same machine code as the Von Neumann computer in the book with the same results and uses almost the same circuitry.

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