Question

How to build a circuit to search for instructions in a ROM memory of a computer system? I don't know how to start doing this homework. Could anyone give me some tips on how to do it?

The computer system stores the first software instruction at address 0 (zero) of a 128x8 ROM memory (128 bits and 8 word bits). The digital instruction fetch system must read an instruction from ROM every clock cycle and decode the 1-byte instruction using a BCD decoder for 7 segments, where for each instruction read it is presented in hexadecimal on two 7-segment displays.
To understand how the instruction search system works, imagine that at address 410 (01002) of ROM memory there is instruction 111010002. Then, in the clock cycle where the program counter returns address 01002, the system will show MSB on the display the value E16 and on the LSB display the value 816.

Part 1: Draw the program counter logic circuit, using flip-flops or logic gates, that allows counting from 00002 to 11112.

Part 2: Design the logic circuit of the 128x8 ROM memory, using logic gates, that allows reading each memory address and outputting the stored byte.

Part 3: Draw the logic circuit of the BCD decoder for two 7-segment displays, which drives the LSB displays, for the four least significant bits, and MSB, for the four most significant bits. Present the truth table and the Karnaugh map.

Part 5: Draw the logic circuit of the entire digital instruction fetch system.

Answer 1

1. Count addresses with a 4-bit counter (e.g., 74HC390).

2. Decode address with 128x8 decoder (e.g., 74HC139) to select RAM chip.

3. Convert ROM data to 7-segment code with BCD decoders (e.g., 74HC4511).

4. Combine outputs in a single circuit, synchronized by clock signal.

Part 1: Program Counter Logic Circuit

* Use a 4-bit counter circuit, such as a 74HC390 Ripple Counter, to count from 0000 (binary) to 1111 (binary).

* Connect the clock input of the counter to the system clock signal.

* Connect the output of the counter to the address decoder input of the ROM.

Part 2: ROM Logic Circuit

* Use a 128x8 decoder, such as a 74HC139 Dual 1-of-8 Decoder, to decode the address from the program counter.

* Connect the outputs of the decoder to the chip select pins of eight 8x8 RAM chips.

* Connect the data lines of each RAM chip to the output data lines of the ROM.

Part 3: BCD Decoder Logic Circuit

* Use a BCD-to-7-segment decoder, such as a 74HC4511 BCD-to-7-Segment Decoder, to convert the binary data from the ROM into a 7-segment code for each display.

* Connect the four most significant bits of the ROM data to the BCD decoder for the MSB display.

* Connect the four least significant bits of the ROM data to the BCD decoder for the LSB display.

Part 4: Digital Instruction Fetch System Logic Circuit

* Combine the outputs of the program counter, ROM, and BCD decoders into a single circuit.

* Use the clock signal to synchronize the operation of the circuit.

Additional Tips

* Use breadboards and jumper wires to prototype the circuit.

* Use a logic simulator to test the circuit before building it.

* Make sure all components are compatible with each other.

I hope these tips help you get started on building your circuit. If you have any other questions, please feel free to ask.

Here are some additional things to keep in mind when building your circuit:

* The specific components you will need will depend on the specific architecture of your computer system.

* The complexity of the circuit will also depend on the number of instructions and the size of the ROM memory.

* It is important to carefully test and debug your circuit to ensure that it is working correctly.