Basic Computer-L1

Basic ComputerORGANIZATIONand Design : 

Basic ComputerORGANIZATIONand Design

IN THIS CHAPTER : 

IN THIS CHAPTER 5-1 Instruction Codes 5.2 Computer Registers 5.3 Computer Instructions 5-4 Timing and Control 5.5 Instruction Cycle 5.6 Memory-Reference Instructions 5.7 Input-Output and Interrupt 5-8 Complete Computer Description 5-9 Design of Basic Computer 5.10Design of Accumulator Logic

5-1 Instruction Codes : 

5-1 Instruction Codes The organization of the computer is defined by its Internal registers. The timing and control structure, The set of instructions that it uses.

Slide 4: 

Although the basic computer presented in this chapter is very small compared to commercial computers, it has the advantage of being simple enough so we can demonstrate the design process without too many complications.

Slide 5: 

A program is a set of instructions that specify the operations, operands, and the sequence by which processing has to occur. A computer instruction is a binary code that specifies a sequence of micro-operations for the computer. Instruction codes together with data are stored in memory. The computer reads each instruction from memory and places it in a control register. The control then interprets the binary code of the instruction and proceeds to execute it by issuing a sequence of micro-operations.

Slide 6: 

An instruction code is a group of bits that instruct the computer to perform a specific operation. It is usually divided into parts, each having its own particular interpretation. The most basic part of an instruction code is its operation part The operation code of an instruction is a group of bits that define such operations as add, subtract, multiply, shift, and complement.

The number of bits required for the operation code : 

The number of bits required for the operation code It depends on the total number of operations available in the computer. The operation code must consist of at least n bits for a given 2n (or less) distinct operations. As an illustration, consider a computer with 64 distinct operations, one of them being an ADD operation. The operation code consists of six bits, with a bit configuration 110010 assigned to the ADD operation.

The relationship between a computer operation and a micro-operation. : 

The relationship between a computer operation and a micro-operation. The control unit receives the instruction from memory and interprets the operation code bits. It then issues a sequence of control signals to initiate microoperations in internal computer registers. For every operation code, the control issues a sequence of microoperations needed for the hardware implementation of the specified operation.

Slide 9: 

An instruction code must specify not only the operation but also the registers or the memory words where the operands are to be found, as well as the register or memory word where the result is to be stored. Memory words can be specified in instruction codes by their address. Processor registers can be specified by assigning to the instruction another binary code of k bits that specifies one of 2k registers.

Stored Program Organization : 

Stored Program Organization The simplest way to organize a computer is to have one processor register and an instruction code format with two parts The first part specifies the operation to be performed and the second specifies an address.

Slide 12: 

Instructions are stored in one section of memory and data in another. For a memory unit with 4096 words we need 12 bits to specify an address since 212 = 4096. If we store each instruction code in one 16-bit memory word, we have available 4 bits for the operation code (abbreviated opcode) to specify one out of 16 possible operations, and 12 bits to specify the address of an operand.

Accumulator (AC) : 

Accumulator (AC) Computers that have a single-processor register usually assign to it the name accumulator and label it AC. The memory-reference operation is performed with the memory operand and the content of AC. If an operation in an instruction code does not need an operand from memory, the rest of the bits in the instruction can be used for other purposes

ADDRESSING MODES : 

ADDRESSING MODES Immediate Addressing Mode When the second part of an instruction code specifies an operand, the instruction is said to have an immediate instruction operand. Direct Addressing mode When the second part specifies the address of an operand, the instruction is said to have a direct address. Indirect addressing mode The second part of the instruction designate an address of a memory word in which the address of the operand is found. One bit of the instruction code can be used to distinguish between a direct and an indirect address.

Computer Registers : 

Computer Registers

Slide 17: 

Computer instructions are normally stored in consecutive memory locations and are executed sequentially one at a time Instruction sequencing needs a counter to calculate the address of the next instruction after execution of the current instruction is completed (PC). It is also necessary to provide a register in the control unit for storing the instruction code after it is read from memory (IR).

Slide 18: 

The data register (DR) holds the operand read from memory The accumulator (AC) register is a general­purpose processing register The temporary register (TR) is used for holding temporary data during the processing.

Registers for the Basic Computer : 

Registers for the Basic Computer

Common Bus System : 

Common Bus System Paths must be provided to transfer information from one register to another and between memory and registers. The number of wires will be excessive if connections are made between the outputs of each register and the inputs of the other registers. A more efficient scheme for transferring information in a system with many registers is to use a common bus.