Timing and control in computer architecture

While public transportation has its benefits, it is not instantaneous. In other words, when you get on a bus, you aren't automatically dropped off at your destination. There are a lot of other factors that determine how long it takes you to get from point A to B. In computer system architecture, a similar situation applies. This image shows the operations undertaken on a system bus: We've talked about raising and lowering flags or signals. Another way to conceptualize the timing diagram is to think of ships at sea. In order to communicate the current activity, flags on the bridge will be raised and lowered. One quick look at the bridge gives the sailors a clear direction of what to do or what not to do. Let's take a step deeper into the architecture and look at a memory read and write. This is a more detailed extract of the timing diagram, showing the write/read operations of the memory: Each interval (vertical dashed lines) is a clock cycle, while the entire flow from left to right is a bus cycle. The bus cycle is the time taken to complete a single read. In this diagram, a single data read/write is taking place. It takes one full bus cycle. For the READ operation, the following actions are undertaken: A bus timing diagram is an architectural design tool that shows the states of bytes as they are transferred through the system bus and memory. The concept is similar to the raising and lowering of flags on a ship: when a flag is raised, a given activity can occur; when lowe...

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Control unit generates timing and control signals for the operations of the computer. The control unit communicates with ALU and main memory. It also controls the transmission between processor, memory and the various peripherals. It also instructs the ALU which operation has to be performed on data. Control unit can be designed by two methods which are given below: Hardwired Control Unit It is implemented with the help of gates, flip flops, decoders etc. in the hardware. The inputs to control unit are the instruction register, flags, timing signals etc. This organization can be very complicated if we have to make the control unit large. If the design has to be modified or changed, all the combinational circuits have to be modified which is a very difficult task. Microprogrammed Control Unit It is implemented by using programming approach. A sequence of micro operations is carried out by executing a program consisting of micro-instructions. In this organization any modifications or changes can be done by updating the micro program in the control memory by the programmer. Difference between Hardwired Control and Microprogrammed Control Hardwired Control Microprogrammed Control Technology is circuit based. Technology is software based. It is implemented through flip-flops, gates, decoders etc. Microinstructions generate signals to control the execution of instructions. Fixed instruction format. Variable instruction format (16-64 bits per instruction). Instructions are regist...

<< Memory-Reference Instructions In order to specify the microoperations needed for the execution of each instruction, it is necessary that the function that they are intended to perform be defined precisely. Some instructions have an ambiguous description. This is because the explanation of an instruction in words is usually lengthy, and not enough space is available in the table for such a lengthy explanation. We will now show that the function of the memory-reference instructions can be defined precisely by means of register transfer notation. Table 5-4 lists the seven memory-reference instructions The decoded D; for i = 0, 1, 2, 3, 4, 5, and 6 from the operation decoder that belongs to each instruction is included in the table. The effective address of the instruc­tion is in the address register AR and was placed there during timing signal T2 when I = 0, or during timing signal T3 when I = 1. The execution of the memory-reference instructions starts with timing signal T4• The symbolic de­scription of each instruction is specified in the table in terms of register transfer notation. The actual execution of the instruction in the bus system will require a sequence of microoperations. This is because data stored in memory cannot be processed directly. The data must be read from memory to a register where they can be operated on with logic circuits. We now explain the operation of each instruction and list the control functions and microoperations needed for their executio...