Microinstruction Formats

Instructions in the form of 0s and 1s will be known as microinstructions. It is the most elementary instruction in the computer, such as moving the contents of a register to the arithmetic logic unit (ALU). It takes several microinstructions to carry out one complex machine instruction. The Microinstruction format are of two types :

  • Horizontal
  • Vertical

1. Horizontal

The control signals are represented in the decoded binary format that is 1 bit/CS. It supports longer control word. It is used in parallel processing applications, because it allows higher degree of parallelism. It requires no additional hardware(decoders). It means it is faster than Verical Microprogrammed. The format of Horizontal microinstruction is as follows : there exist a one bit for each internal processor control line and one bit for each system bus control line. There is a condition field denoting the condition under which a branch action is to be performed, and there exist a field possessing the address of the microinstruction to be executed next when a branch is teken. Such a microinstruction is understand as follows : 

Figure : Horizontal Microinstruction

a. To execute this microinstruction, turn on all the control lines denoted by a 1 bit; leave off all control lines denoted by a 0 bit. The resulting control signals will produce one or more micro-operations to be done.

b. If the condition denoted by the condition bits is false, execute the next microinstruction present in sequence.

c. If the condition denoted by the condition bits is true, the next microinstruction to be executed is denoted in the address field

2. Vertical

In Vertical microinstructions the control signals are represented in the encoded binary format. For N control signals- Log2(N) bits are required. It supports shorter control words and it is more flexible because it supports easy implementation of new control signals. The vertical microinstructions Requires an additional hardware (decoders) to generate control signals, it implies it is slower than horizontal microprogrammed. 

Figure : Vertical Microinstruction

The format of microinstruction or control word as above given, is same as horizontal microinstruction except that the function code is used to perform each action. 

The above figure represent how these control words or microinstructions may be organized in a control memory. The microinstructions in each routine are executed sequentially according to the order of occurence. Each routine finishes with a branch or jump instruction denoting where to go next. There is an important execute cycle routine whose only purpose is to represent that one of the machine instruction routines is to be executed next, according to the current opcode.

The control memory of the above figure is to the point explanation of the complete operation of control unit. It defines the sequence of micro-operations to be done during each cycle (fetch,indirect, execute, interrupt), and it denotes the sequencing of these cycles. These representation would be important device for documenting the functioning of a control unit for a specific computer. It is also a way of implementing the control unit.

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