Lecture 9: Basic Computer Organisation

#ComputerOrganisation #Memory #Registers #AddressingModes #InstructionFormat
THE BASIC COMPUTER
The Basic Computer has two components, a processor and memory
The memory has 4096 words in it and each word is 16 bits long.

Execution of Instruction:
The instructions of a program, along with any needed data are stored in memory. The CPU reads the next instruction from memory.It is placed in an Instruction Register (IR).Control circuitry in control unit then translates the instruction into the sequence of microoperations necessary to implement it

INSTRUCTION FORMAT
A computer instruction is often divided into two parts
An opcode (Operation Code) that specifies the operation for that instruction
An address that specifies the registers and/or locations in memory to use for that operation
In the Basic Computer, since the memory contains 4096 words, we needs 12 bit to specify which memory address this instruction will use. In the Basic Computer, bit 15 of the instruction specifies the addressing mode (0: direct addressing, 1: indirect addressing)
Since the memory words, and hence the instructions, are 16 bits long, that leaves 3 bits for the instruction’s opcode

ADDRESSING MODES
The address field of an instruction can represent either
Direct address: the address in memory of the data to use (the address of the operand), or
Indirect address: the address in memory of the address in memory of the data to use

Effective Address (EA)
The address, that can be directly used without modification to access an operand for a computation-type instruction, or as the target address for a branch-type instruction

REGISTERS
A processor has many registers to hold instructions, addresses, data, etc
The processor has a register, the
Program Counter (PC) that holds the memory address of the next instruction to get executed. Since the memory in the Basic Computer only has 4096 locations, the PC only needs 12 bits.
In a direct or indirect addressing, the processor needs to keep track of what locations in memory it is addressing: The Address Register (AR) is used for this. The AR is a 12 bit register in the Basic Computer
When an operand is found, using either direct or indirect addressing, it is placed in the Data Register (DR). The processor then uses this value as data for its operation
The Basic Computer has a single general purpose register – the Accumulator (AC)
The significance of a general purpose register is that it can be referred to in instructions
e.g. load AC with the contents of a specific memory location; store the contents of AC into a specified memory location
Often a processor will need a scratch register to store intermediate results or other temporary data; in the Basic Computer this is the Temporary Register (TR)
The Basic Computer uses a very simple model of input/output (I/O) operations
Input devices are considered to send 8 bits of character data to the processor
The processor can send 8 bits of character data to output devices
The Input Register (INPR) holds an 8 bit character gotten from an input device
The Output Register (OUTR) holds an 8 bit character to be send to an output device

[studentknowledge]

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