# Combinatorial and Sequential Circuits

Posted on Feb 5, 2014

To the left in the diagram, you find the micro memory containing up to 64 different addresses each with 15 bits (numbered 1 to 15) that we call the micro operations or MOPs for short. The MOPs control every aspect of the computer. To the left, you also find the micro PC, which is a counter register with clear. The width of the micro PC is 6 bits

for a total of 64 different values. The micro PC is controlled by MOPs number 1 and 2. When MOP number 1 has the value 1, the micro PC will be cleared after the next clock pulse. Similarly, MOP number 2 will cause a new value to be loaded into the micro PC from the instruction decoder. The micro PC can also be cleared manually from the reset button. Since the The final item to the left of the data bus is the instruction decoder. Its job is to translate instruction codes to addresses in the micro memory where the micro program for the instruction starts. The input to the instruction decoder is the low 5 bits of the data bus, and the output is 6 bits wide. The lower part of the diagram is the arithmetic unit with two general-purpose registers, R0 and R1. Whether R0 is loaded from the data bus is determined by MOP number 11. Whether R1 is loaded from the output of the ALU is controlled by MOP number 12. The ALU uses MOPS 13 to 15 to control what operation is to be performed. The inputs (8 bits wide) are taken from the registers R0 and R1 and the output (also 8 bits wide for now) goes to the input of R1. The output of R1 can be connected to the data bus by MOP number 8 which is connected to a bus driver (BD) which in effect closes the connection from the output of R1 to the data bus. input is controlled by MOP number 4. When MOP number 4 has the value 1, the memory is written. The address of the memory is taken from the address...

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