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Circuit Description

This applet demonstrates the D*CORE microcode sequencer, which is the heart of the D*CORE processor control-unit. This applet is used in the context of the T3 laboratory course to introduce the basics of a microcoded control-unit.

The circuit consists of three major parts, namely the microcode memory (uROM), the microcode program counter register (MPC), and some feedback logic that decides the next microprogram address. The 4:1 multiplexer is controlled by two bits of the current microcode output word, and selects one of four different input values:

• leftmost, an external input
• second, the output of the 2:1 multiplexer, which in turn is controlled by the xs input switch. In the complete processor, the xs-signal is connected to the carry-flag and used for conditional jumps.
• the nextA value from the current microcode word,
• the nextB value from the current microcode word.

Open the microcode memory editor and edit the microprogram words to create an interesting microcode sequence with at least five or six states. Double-click the microcode single-bits to switch the bits, or type in the new values directly.

For details, check the course material (in German) on our webserver.