Basic Computer Organization and Design

Basic Computer Registers

Computer instructions are normally stored in the consecutive memory locations and are executed sequentially one at a time. Thus computer needs processor registers for manipulating data and holding memory address which are shown in the following table:

• Since the memory in the Basic Computer only has 4096 (=2¹²) locations, PC and AR only needs 12 bits.
• Since the word size of Basic Computer only has 16 bit, the DR, AC, IR and TR needs 16 bits. 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 and the Output Register (OUTR) holds an 8-bit character to be sent to an output device.

Common Bus System

• The basic computer has eight registers, a memory unit, and a control unit.
• These registers, memory and control unit are connected using a path (bus) so that information can be transferred to each other.
• If separate buses are used for connecting each registers, it will cost high.
• The cost and use of extra buses can be reduced using a special scheme in which many registers use a common bus, called common bus system.

• Three control lines S2, S1 and S0 control the register to be selected as the input by the bus.

S2	S1	S0	Register
0	0	0	X (nothing)
0	0	1	AR
0	1	0	PC
0	1	1	DR
1	0	0	AC
1	0	1	IR
1	1	0	TR
1	1	1	Memory

• The lines from the common bus are connected to the inputs of each register and the data inputs of the memory.
• The particular register whose LD (load) input is enabled receives the data from the bus during the next clock pulse transition.
• The memory receives the contents of the bus when its write input is activated.
• The memory places its 16 bit output onto the bus when the read input is activated and S2S1S0 = 111.

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