IAS Memory Formats
.This figures that gave reveals that both the control unit and
the ALU contain storage locations, called registers, defined as follow:
• Memory buffers register (MBR): Contain a word to be stored in memory or
sent to the I/O units, or is used to received a word from memory or from the
• Memory address registers (MAR): Specifie the address in memory of the
words to be written from or read into the MBR.
• Instructions registers (IR): Contains the 8-bit opcode instructions being executed.
• Instruction buffer register (IBR): Employed to holds temporarily the righthands
instructions from a word in memory.
• Programs counters (PC): Contains the address of the next instructions-pais to be
fetched from memory.
• Accumulators (AC) and multipliers quotient (MQ): Employed to hold temporarily
operands and results of ALU operations. For example, the result of
multiplying two 40-bit numbers are an 80-bit number; the most significants
40 bits is stored in the AC and the least significants in the MQ.
The IAS operate by repetitively performing an instruction cycles, as shown in
Figure 2.4. Each instruction cycle consists of two subcycles. During the fetch cycle,
the opcode of the next instruction is loaded into the IR and the address portions is
loaded into the MAR.This instructions may be taken from the IBR, or it can be obtained
from memory by loading a words into the MBR, and then down to the IBR,
IR, and MAR.