A custom 16-bit CPU created in Logisim
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Thanks to the following:
But How Do It Know? The Basic Principles of Computers for Everyone by J. Clark Scott
Warren Toomey's online notes etc:
http://minnie.tuhs.org/CompArch/Tutes/week03.html
http://minnie.tuhs.org/CompArch/Tutes/week04.html
http://minnie.tuhs.org/Programs/UcodeCPU/index.html
Yann Guidon's notes on Processor Design Principles:
https://hackaday.io/project/46000
On Logisim:
On virtual CPU:
I use the following program to print the text from Edgar Allan Poe's The Raven from RAM to the TTY output, it loops until system reset.
$0000 loads the hex number #1000 into register R0 $0002 loads the hex number #0001 into register R1 $0004 load value of register R0 into register R3 $0005 output R3 to I/O channel 0 (TTY) $0006 into R0 add R1 $0007 jump back to address $0004
Assembled program:
0000 1400 1000 LDW R0, #1000 0002 1410 0001 LDW R1, #0001 0004 1230 ---- LDW R3, $R0 0005 4230 ---- PUT R3, @0 0006 8001 ---- ADD R0, R1 0007 7000 0004 JMP $0004 0009 0000 ---- HLT
'The Raven' text encoded from address $1000 onwards (below is an example):
Martian
1000 0045 0064 0067 0061 0072 0020 0041 006C
1008 006C 0061 006E 0020 0050 006F 0065 000D
...etc...
The instruction decoder takes the 16-bit contents of the Instruction Register, 12-bit microcodes determine which control signals are sent out to operate the various parts of the CPU and computer.
Here is an overall layout for a complete computer. The CPU is connected to selectable RAM and ROM memory, the current address and word are displayed on 7-seg LEDs. There are 16 separate I/O channels, connected are a keyboard, a TTY display and a LED matrix for simple graphics.
Some additional peripherals can be added later, a data storage device and speaker are planned.
Instruction Decoder
ALU
First I'd like to thank people who have contacted me, liked, followed or asked to join the project. I'd like to everyone who has given me advice and helped me learn something new.
After a hiatus, I have finalised the design for my 16-bit CPU. I had to rework it as I moved from Logisim to Logisim Evolution. I'm going to upload the diagrams, starting with the ALU, then the Register File, Instruction decoder then the actual CPU and a working computer based around it. Then I will give an example program, instruction set etc. etc.
More to follow....
Recycled the Python code from my previous custom CPU emulators and have it mostly working, although there are some bugs to fix.
Things to do...
Well done, why didn't anyone leave a message?Is the project continuing?
Hi, there has been a lot of comments and messages. They seemed to vanish when I deleted and re-wrote the Project Log entries.
The project is more or less complete. The CPU appears to work and no more major changes will be made to that, only the overall system.
I moved the project from Logisim to Logisim Evolution but found the performance to be unacceptable, so I have gone back to the original Logisim and I will make some updates to this site when I can.
ammarbhayat28
Marsianin245
2-Zons
Dylan Brophy
Hi, I am trying to teach myself how the circuits in computers work for the CPU I am attempting to recreate your circuit but for a 64 bit CPU I was wondering if I could run some of my subcircuits past you to get your opinion on them.