L10:Computer Architecture

8- DATAPATH
As mentioned, the CPU can be divided into a data section and a control section. The data section, which is also called the datapath, contains the registers and the ALU. The datapath is capable of performing certain operations on data items.
The control section is basically the control unit, which issues control signals to the datapath. Internal to the CPU, data move from one register to another and between ALU and registers. Internal data movements are performed via local buses, which may carry data, instructions, and addresses. Externally, data move from registers to memory and I/O devices, often by means of a system bus. Internal data movement among registers and between the ALU and registers may be carried out using different organizations including one-bus, two-bus, or three-bus organizations. Dedicated datapaths may also be used between components that transfer data between themselves more frequently. For example, the contents of the PC are transferred to the MAR to fetch a new instruction at the beginning of each instruction cycle. Hence, a dedicated datapath from the PC to the MAR could be useful in speeding up this part of instruction execution.
8-1 One-Bus Organization
Using one bus, the CPU registers and the ALU use a single bus to move outgoing and incoming data. Since a bus can handle only a single data movement within one clock cycle, two-operand operations will need two cycles to fetch the operands for the ALU. Additional registers may also be needed to buffer data for the ALU.
This bus organization is the simplest and least expensive, but it limits the amount of data transfer that can be done in the same clock cycle, which will slow down the overall performance. Figure (8.1) shows a one-bus datapath consisting of a set of general-purpose registers, a memory address register (MAR),
Computer Architecture …………………………