Toy CPU
A simulation of a Minimal Instruction Set Computer
Status
Working version (FreeDOS graphical mode)
License
MIT
History
I teach university courses part-time, and one course that I teach is MIS 100, where students learn how technology works. For our section on "programming," I usually talk about programming in very abstract terms, because these are not Computer Science students. But this year, I wanted to try something new.
I hoped to start the "programming" discussion by walking my students
through a history of programming, so they could see the next step
and how that worked. I tried to find a simple hobby "educational"
computer, similar to the Altair 8800, where you input a series of program
instructions in "machine language" (binary opcodes) using switches and
LEDs. I wanted the instructions to be simple enough that my students could
write a few simple programs, like A+B=C, and use that as a starting
point to write the same program in Assembly, and in C, and ... you get
the idea.
But I couldn't find a suitable "Altair-like" SBC for less than $100. There are "Altair" software emulators out there, but they faithfully reproduce the Altair 8800, and that was too much overhead for my needs.
So I decided to write my own hobby "educational" computer. I call it the Toy CPU.
I wrote a rough prototype on DOS using character mode, but you couldn't enter a program on the "front panel." Instead, you had to hard-code a program into memory, and recompile to run that. It was very primitive, but enough to know that it worked.
Later, I completely rewrote the Toy CPU using Linux ncurses. This was still a prototype, but this time you could enter a program on the "front panel" by "flipping bits." It looked okay in ncurses, but I really wanted a graphical program.
Open Jam 2022 came up at the right time! I decided to completely rewrite the Toy from scratch, using FreeDOS and other open source tools. I don't use graphical assets per se; instead, the Toy draws the interface elements using basic graphical functions from OpenWatcom (open source). If you're curious: I used FED (open source) as my editor. I ran FreeDOS inside VirtualBox (open source) running on top of Fedora Linux (open source). Everything in the pipeline was open source.
The theme for Open Jam 2022 is "Light in the Darkness," which is a perfect fit for the Toy CPU, because of all the blinkenlights!
How it works
I intentionally kept this as a very simple implementation. My goals were to make it easy to write and easy to understand.
The Toy CPU implements 256 bytes of program memory, and an accumulator. You program the Toy using binary opcodes. When you run a program, the Toy CPU starts at counter zero for the first instruction.
Programming
When you start the Toy, it will "boot up" by clearing all the values in the 256-byte memory. You'll see it count up from 0 to 255 in the counter display, while the instruction and accumulator displays remain at zero.
Look at the status display on the bottom-right of the Toy. You will see "PWR" when the Toy is turned on, and "INI" when the Toy is initializing.
After initialization, you'll be put into Input mode. Look on the status display and you will see "INP" light up to indicate you are in Input mode.
In Input mode, you enter a program using "switches and lights" on the front panel, similar to computers in the 1960s or 1970s. Use the Up/Down arrow keys to select the instruction you want to edit, then press Enter to edit an instruction.
Look on the status display, and you will see "EDT" light up to show you are in Edit mode.
In Edit mode, use the Left/Right arrow keys to select a bit in the instruction, and hit Space to flip a specific bit in an instruction. When you're done entering an intruction, press Enter to go back to Input mode.
From Input mode, press R to run the program. This always runs the
program from the first instruction. The status display will light up
"RUN" when the Toy runs your program.
If you need to abort your program, press the Esc key during program execution and the Toy will light up the "ABT" in the status display before it drops you back into Input mode.
To exit the Toy, press Q while in Input mode. The Toy will light up the
"HLT" light on the status and then will quit to DOS.
Here are the opcodes for the Toy CPU, as currently implemented:
| opcode | Name | Description |
|---|---|---|
| 00000000 | STOP | Stop program execution. |
| 00000001 | RIGHT | Bit shift the accumulator to the right by 1 position. The value 00000010 becomes 00000001, and the value 00000001 becomes 00000000. |
| 00000010 | LEFT | Bit shift the accumulator to the left by 1 position. The value 00000010 becomes 00000100, and 10000000 becomes 00000000. |
| 00001111 | NOT | Binary NOT the accumulator. The value 10101111 becomes 01010000. |
| 00010001 | AND | Binary AND the accumulator with the value stored at an address. |
| 00010010 | OR | Binary OR the accumulator with the value stored at an address. |
| 00010011 | XOR | Binary XOR the accumulator with the value stored at an address. |
| 00010100 | LOAD | Load the accumulator with the value stored at an address. |
| 00010101 | STORE | Copy the accumulator into an address. |
| 00010110 | ADD | Add the value stored at an address to the accumulator. |
| 00010111 | SUB | Subtract the value stored at an address from the accumulator. |
| 00011000 | GOTO | Jump to a program address. |
| 00011001 | IFZERO | If the accumulator is zero, jump to a program address. |
| 10000000 | NOP | No operation; safely ignored. |
Any other unrecognized instruction is the equivalent of a NOP. However,
do not rely on other opcodes being the same as NOP. For example, in
the current version of the Toy, any instruction with ...1.... will
also perform an extra fetch operation.
Sample programs
Flash the lights
It helps to understand the Toy CPU by looking at a sample program. In this example, we'll "flash" the accumulator lights. First we'll light up the lower bits, then the higher bits, then all 8 bits. This tests sequential operation of the Toy:
0. LOAD
1. "A" (7)
2. LOAD
3. "B" (8)
4. LOAD
5. "C" (9)
6. STOP
7. "A" = 00001111
8. "B" = 11110000
9. "C" = 11111111
This would be entered into the Toy like this:
(counter: 00000000) instruction: 00010100
(counter: 00000001) instruction: 00000111
(counter: 00000010) instruction: 00010100
(counter: 00000011) instruction: 00001000
(counter: 00000100) instruction: 00010100
(counter: 00000101) instruction: 00001001
(counter: 00000110) instruction: 00000000
(counter: 00000111) instruction: 00001111
(counter: 00001000) instruction: 11110000
(counter: 00001001) instruction: 11111111
This is a series of LOAD instructions that first loads the value
00001111 into the acccumulator, effectively lighting up the right side
of the accumulator. Then it loads the value 11110000, lighting up the
left side of the accumulator. Finally, it loads the value 11111111,
lighting up all the lights on the accumulator.
Flash the lights (alternate method)
A more efficient way to write this program is to use logical operators to
operate on the initial 00001111 value. This loads 00001111 into the
accumulator, then performs a logical NOT, resulting in 11110000. Then
it uses OR with the original 00001111 value to get 11111111 before
performing another NOT to give the final 00000000 result:
0. LOAD
1. "A" (7)
2. NOT
3. OR
4. "A" (7)
5. NOP
6. STOP
7. "A" = 00001111
If you want to edit a program that's already entered into memory, you may
need to use NOP statements to skip over now-unused instructions. In this
example, I modified the previous "Flash the lights" program, which had
STOP at instruction 6. Since the updated program ends at instruction 4,
I added a NOP statement at instruction 5 so the program would flow to
the STOP at instruction 6.
Move a light
Or consider this sample program that moves a single light from the left to the right:
0. LOAD
1. "A" (8)
2. RIGHT ("loop start")
3. IFZERO
4. "end" (7)
5. GOTO
6. "loop start" (2)
7. STOP ("end")
8. "A" = 10000000
To make this program easier to read, I've added notes in parentheses for
the "loop start" at instruction 2, and "end" at instuction 7. I always
write out my programs on paper first, and using this notation makes it
easier to go back later and fill in the instruction values. For example,
use the value 00000111 (7) at instruction 4, and the value 00000010
(2) at instruction 6.
Countdown
Or this sample program that counts down from 15:
0. LOAD
1. "A" (9)
2. SUB ("loop start")
3. "one" (10)
4. IFZERO
5. "end" (8)
6. GOTO
7. "loop start" (2)
8. STOP ("end")
9. "A" = 00001111 (15)
10. "one" = 00000001 (1)
A + B = C
Consider this short program that adds two values, "A" and "B," and stores the result in the variable "C."
0. LOAD
1. "A" (7)
2. ADD
3. "B" (8)
4. STORE
5. "C" (9)
6. STOP
7. "A" = some number
8. "B" = some other number
9. "C" = placeholder
If you enter and run this program, you should see instruction 9 stores the result of adding "A" and "B" together. It will be the same value as the accumulator when the program has stopped running. Note that whatever value was stored in "C" beforehand will be overwritten.
Compare A and B
Or this sample program that compares two numbers, "A" and "B." If they
are the same, the accumulator displays 00000000. If they are different,
the accumulator displays 11111111:
0. LOAD
1. "A" (9)
2. XOR
3. "B" (10)
4. IFZERO
5. "end" (8)
6. LOAD
7. "X" (11)
8. STOP ("end")
9. "A" = some number
10. "B" = some other number
11. "X" = 11111111