Computer instructions are normally stored in consecutive memory locations and are executed sequentially one at a time. The control reads an instruction from a specific address in memory and executes it. It then continues by reading the next instruction in sequence and executes it, and so on. This type of instruction sequencing needs a counter to calculate the address of the next instruction after execution of the current instruction is completed. It is also necessary to provide a register in the control unit for storing the instruction code after it is read from memory. The computer needs processor registers for manipulating data and a register for holding a memory address. These requirements dictate the register configuration shown in Fig. 5-3. The registers are also listed in Table 5-1 together with a brief description of their function and the number of bits that they contain.
The memory unit has a capacity of 4096 words and each word contains 16 bits. Twelve bits of an instruction word are needed to specify the address of an operand. This leaves three bits for the operation part of the instruction and a bit to specify a direct or indirect address. The data register (DR) holds the operand read from memory. The accumulator (AC) register is a generalpurpose processing register. The instruction read from memory is placed in the instruction register (IR). The temporary register (TR) is used for holding temporary data during the processing.