In vertical microcode, each microinstruction is encoded i.e., the bit field can pass through intermediate combinatory logic which in turn creates the actual control signals for internal CPU components (ALU, registers, etc). In vertical microcode, the micro-operations are grouped into fields such that no more than one micro-operation in a field is active during any state. Then a unique field value is assigned to each micro-operation in the field.For example, a field with eight different micro-operations would require 3 bits, each value from 000 to 111 would be assigned to one of the eight micro-operations. The micro-operation field bits are ... Read More

In horizontal microcode, each micro-operation is represented by one bit in each microinstruction. Horizontal microcode is generally included in a fairly wide control save it is not exceptional for each work to be 56 bits or more. On each click of a sequencer clock, a microcode word is read, decoded, and used to control the functional components which create up the CPU. The micro-operations and their mnemonics are shown in the table.Microoperations and their mnemonics for the very simple CPUMnemonicMicro-OperationARPCAR←PCARDRAR←DR[5….0]PCINPC←PC+1PCDRPC←DR[5…0]DRMDR←MIRDRIR←DR[7…6]PLUSAC←AC+DRANDAC←AC ^ DRACINAC←AC+1Since there are nine micro-operations, each word of microcode requires 9 bits to represent them, 1 bit per micro-operation. ... Read More

The microprogram sequence is a general-purpose building block for the microprogrammed control unit. The main objective of the microprogram sequencer is to demonstrate an address to the control memory so that microinstruction can be read and executed. The next address logic of the sequencer decides the particular address source to be loaded into the control address register. The figure shows the layout of the simple microsequencer.There are only two possible next addresses are used − the opcode mapping and an absolute jump. The last state of the fetch cycle, FETCH3, goes to one of the four execute routines. This must ... Read More

A microsequencer is also designed as a finite state machine. Consider the generic microsequencer shown in the figure. The register stores a value that corresponds to one state in the CPU’s state diagram. It serves as the address that is input to the microcode memory. This memory outputs a microinstruction, the contents of the memory location for that address.The microinstruction consists of a several-bit field, which can be broken into two groups. The first group is the microoperations. These signals are output from the microsequencer to the rest of the CPU. The second group of bits of the microinstruction is ... Read More

The status register comprises the status bits. The bits of the status register are modified according to the operations performed in the ALU. The figure displays a block diagram of an 8-bit ALU with a 4-bit status register.If the end carry C8 is 1, then carry (C) is set to 1. If C8 is 0, then C is cleared to 0.If the highest order bit F7 is 1, then Sign (S) is set to 1. If F7 is 0, then S is set to 0.If the output of ALU is 0, then zero (Z) is set to 1, otherwise, Z ... Read More

Instructions of the computer are always stored in consecutive memory locations. These instructions are fetched from successive memory locations for processing and executing.When an instruction is fetched from the memory, the program counter is incremented by 1 so that it points to the address of the next consecutive instruction in the memory. Once a data transfer and data manipulation instruction are executed, the program control along with the program counter, which holds the address of the next instruction to be fetched, is returned to the fetch cycle.Data transfer and manipulation instructions specify the conditions for data processing operations, whereas the ... Read More

Data manipulation instructions have computational capabilities. They perform arithmetic, logic, and shift operations on data.There are three types of data manipulation instructions are as follows −Arithmetic InstructionsArithmetic operations include addition, subtraction, multiplication, and division. Some computers provide instructions only for addition and subtraction operations and generate multiplication and division operations from these two operations. Each instruction is represented by a mnemonic symbol.The table shows some of the arithmetic instructions and their respective mnemonic symbols.Arithmetic InstructionsNameMnemonic SymbolsLoadLDStoreSTMoveMOVExchangeXCHInputInOutputOUTPushPUSHPopPOPThe description of these instructions is as follows −Increment − The increment instruction adds 1 to the value stored in the register or memory word.Decrement ... Read More

Data transfer instructions transfer the data between memory and processor registers, processor registers, and I/O devices, and from one processor register to another. There are eight commonly used data transfer instructions. Each instruction is represented by a mnemonic symbol.The table shows the eight data transfer instructions and their respective mnemonic symbols.Data Transfer InstructionsNameMnemonic SymbolsLoadLDStoreSTMoveMOVExchangeXCHInputInOutputOUTPushPUSHPopPOPThe instructions can be described as follows −Load − The load instruction is used to transfer data from the memory to a processor register, which is usually an accumulator.Store − The store instruction transfers data from processor registers to memory.Move − The move instruction transfers data from ... Read More

The operands of the instructions can be located either in the main memory or in the CPU registers. If the operand is placed in the main memory, then the instruction provides the location address in the operand field. Many methods are followed to specify the operand address. The different methods/modes for specifying the operand address in the instructions are known as addressing modes.Types of Addressing ModesThere are various types of Addressing Modes which are as follows −Implied Mode − In this mode, the operands are specified implicitly in the definition of the instruction. For example, the instruction "complement accumulator" is ... Read More

Instructions in a computer can be of multiple lengths with a variable number of addresses. The various address fields in the instruction format of a computer vary as per the organization of its registers. It depends on the multiple address fields the instruction can be categorized as three address instructions, two address instructions, one address instruction, and zero address instruction.Three Address InstructionsThe general format of a three address instruction is defined as −operation source 1, source 2, destinationADD A, B, Cwhere A, B, and C are the three variables that are authorized to a different area in the memory. ‘ADD’ ... Read More