8-bit CPU control logic: Part 2

Summary

This video script discusses the process of executing a computer program by manually setting control signals for each micro-instruction. It explains how to use a 74 LS161 4-bit counter to track micro-instruction steps and a 74 LS138 decoder to translate binary values into control signals. The script also explores optimizing instruction execution by combining certain steps and using an EEPROM for combinational logic, aiming to efficiently manage the computer's instruction cycle.

Takeaways

• 💻 The video discusses executing a program on a computer by manually setting control signals for micro instructions.
• 📝 A program was written to load data from memory address 14 (containing 28), add the contents of memory address 15 (containing 14), and output the result.
• 🔍 Each instruction is composed of several micro instructions, which are controlled by setting control words high or low.
• 🕒 The process involves stepping through the program by pulsing the clock and setting control words for each micro instruction.
• 🔢 A 74 LS 161, a 4-bit counter, is used to keep track of the micro instruction steps within an instruction cycle, counting up to 16 micro instructions.
• 🔗 The counter's outputs are used to light up LEDs, visually representing the count and helping to track the current micro instruction.
• 🔄 An inverter is used to create a secondary clock that stays in sync with the main clock but updates the control logic just before the main clock pulses.
• 📺 A 74 LS 138 decoder is used to translate the binary count from the counter into separate signals indicating the current micro instruction step.
• 🔧 The video demonstrates a method to optimize instruction execution by combining certain steps, reducing the number of micro instructions needed for complex operations.
• 🛠️ Future videos will cover more sophisticated methods of organizing control logic, including the use of EEPROMs for combinational logic and microprogramming to define computer instructions.

Q & A

What is the purpose of the program discussed in the video?

The program's purpose is to load data from Memory address 14 into the a register, add the contents of Memory address 15 to it, and then output the result of the addition.

What are micro instructions?

Micro instructions are the individual steps that make up a larger instruction in a computer program. Each step is executed in sequence to perform the overall instruction.

How does the control logic set up the control word for each micro instruction?

The control logic sets up the control word for each micro instruction by knowing which instruction is being executed and which step in the process is currently being performed.

Why is it necessary to know the current step in the micro instruction sequence?

Knowing the current step is necessary to set up the control word appropriately for each micro instruction, as the first three steps are the same for every instruction, but the subsequent steps vary based on the specific instruction being executed.

What is the role of the 74 LS 161 in the control logic?

The 74 LS 161 is a 4-bit counter used in the control logic to count the steps in the micro instruction cycle, allowing the system to keep track of the current step.

How does the inversion of the main clock help in the control logic?

Inverting the main clock provides a secondary clock signal that is out of phase with the main clock, allowing the control logic to update just before the main clock pulses, which is necessary for setting up the control word before the main execution of each step.

What is the function of the 74 LS 138 in the control logic?

The 74 LS 138 is a decoder used to translate the binary count from the 74 LS 161 counter into separate signals that indicate the current micro instruction step.

Why is it beneficial to combine the RAM out and instruction register in steps with the counter enable?

Combining these steps saves a micro instruction cycle, reducing the total number of steps required for each instruction, which in turn makes the overall execution of instructions faster.

What is the significance of the counter resetting at a specific step?

Resetting the counter at a specific step ensures that the control logic only needs to count up to the maximum number of steps required for the most complex instruction, optimizing the use of the counter and speeding up instruction execution.

How does the video suggest organizing the control logic for the computer?

The video suggests using a combination of counters, decoders, and EEPROMs to organize the control logic. EEPROMs can be programmed to handle the micro instructions based on the counter and instruction register inputs, allowing for a flexible and efficient control logic system.