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A tiny CPU
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Calculus Vaporis CPU Design

This is a sketch of a design for a very small zero-operand 12-bit CPU in about 1000 NAND gates, or a smaller number of more powerful components (e.g. 70 lines of C). It’s my first CPU design, so it may be deeply flawed.

Initially I called it the “Dumbass CPU”, but I thought that didn’t seem like a name Charles Babbage would have used. So I called it “Calculus Vaporis”, which means “counting-stone of steam” in Latin, I hope.

This directory contains a simulator written in C, a few simple programs in a simple assembly language, and an assembler written in Python. Run make to try them out.

Logical Organization

The machine has four 12-bit registers: P, A, X, and I.

P is the program counter. It generally holds the address of the next instruction to fetch. It could actually be only 11 bits.

A is the top-of-stack register.

X is the second-on-stack register.

I is the instruction-decoding register. It holds the instruction word currently being decoded.

The machine additionally has an 11-bit address bus B, a 12-bit data bus D, and a bus operation O, which are treated as pseudo-registers for the purpose of the RTL description. The narrow memory bus is weird but simplifies the usage of immediate constants.

Instruction Set

The machine cycle has four microcycles: fetch 1, fetch 2, execute 1, execute 2. Fetch 1 and 2 are the same for all instructions. All instructions but @ do nothing during execute 2. The other RTL below explains what happens during execute 1.

Following Python, I am using R[-1] to refer to the highest bit of register R, R[:-1] to refer to all but the highest bit, R[-2] to refer to the next-highest bit, and so on.

There are seven instructions: $, ., -, |, @, !, and nop. They are stored one per machine word, which is grossly wasteful of memory space but makes for a simpler instruction decode cycle. Avoiding the gross waste of memory space would probably require adding at least one more register to the processor, because as it is defined now, roughly half the instructions in any code are $, which needs a whole machine word most of the time anyway.

There are even simpler instruction sets, such as the subtract- indirect-and-branch-indirect-if-negative OISC, and the MOV machine. While these instruction sets are simpler to explain, the existing decoding logic is fairly small, about 20 out of the 1000 NAND gates. These simpler instruction sets call for a more complicated machine cycle.

  • $ is the “load immediate” instruction; it’s represented by having a single high bit set in a 12-bit word. When written in code sequences, it is followed by the value of the other 11 bits. It does:

    X ← A, A[:-1] ← I[:-1], A[-1] ← I[-2]

    Note that this is the only way to change the value of X, by copying A’s old value into it.

  • . is the “conditional call/jump” instruction. It does:

    if X[-1] == 0: 
        P ← A[:-1], A[:-1] ← P, A[-1] ← 0;
        A ← X;  # not very important, kind of arbitrary, maybe drop it
  • - is the “subtract” instruction. It does:

    A ← X - A;
  • | is the “NAND” instruction (named after the Sheffer stroke). In C notation for bitwise operators, it does:

    A ← ~(A & X);
  • @ is the “fetch” instruction. It is the only instruction that needs both execute microcycles. During execute 1, it does:

    B ← A[:-1], O ← “read”;

    Then, during execute 2:

    A ← D;
  • ! is the “store” instruction. It does:

    B ← A[:-1], D ← X, O ← “write”, A ← X;
  • nop does nothing.

Code Snippets

These show that the instruction set is usable, just barely.

Call the subroutine at address x, passing the return address in A:

$0 $x .

Store the return address passed in A at a fixed address ra (for non-reentrant subroutines):

$ra !

Return to the address thus passed, trashing both A and X:

$0 $ra @ .

Decrement the memory location sp, used as, for example, a stack pointer:

$sp @ $1 - $sp !

Store a return address at the location pointed to by sp after decrementing sp, using an address tmp for temporary storage:

$tmp !  $sp @ $1 - $sp !  $tmp @ $sp !

Fetch that return address from the stack, increment sp, and return to the address:

$sp @ @ $tmp !  $sp @ $-1 - $sp !  $0 $tmp @ .

Negate the value stored at a memory location var:

$0 $var @ - $var !

Add the values stored at memory locations a and b with the aid of a third temporary location tmp, leaving the sum in the A register:

$0 $a @ - $tmp ! $b @ $tmp @ -

The rest of the RTL

The instruction definitions above define what happens at the RTL level during the instruction execution microcycles. The RTL for the other two microcycles of the machine cycle follow:

Fetch 1:

> B ← P, P ← P + 1, O ← “read”;

Fetch 2:

> I ← D;

The presumed memory interface semantics are:

When O ← “read”, on the next microcycle:

> D ← M[B];

When O ← “write”:

>  M[B] ← D.

I don’t know how enough about memory to know how realistic that is.

Translating the RTL design into gates

(This part contains a number of errors. Hopefully it’s accurate enough that the number of gates can be meaningfully estimated.)

We need a multiplexer attached to the input of most of the registers to implement the RTL described earlier. Here are the places each thing can come from, and when:

register is set from when
A I[:-1] sign-extended ($, execute 1)
P (., execute 1, if X[-1] == 0)
X - A (-, execute 1)
!(X & A) (|, execute 1)
D (@, execute 2)
X (!, execute 1; or ., execute 1, when X[-1] == 1)
X A ($, execute 1)
P P + 1 (fetch 1)
A (., execute 1, if X[-1] == 0)
I D (fetch 2)
B P (fetch 1)
A[:-1] (@ or !, execute 1)
D X (!, execute 1)
O “read” (fetch 1, or @, execute 1)
"write" (!, execute 1)

At all other times, registers continue with their current values.

The overall design, then, looks something like this. Some of the “wires” are N bits wide:

    register(P_output, P_input, P_write_enable)
    register(A_output, A_input, A_write_enable)
    register(X_output, X_input, X_write_enable)
    register(I_output, I_input, I_write_enable)
    instruction_decoder(I_output[-4:], fetch_enable, instruction_select)
    # everything up to execute_1 is the inputs; everything after
    # that is an output
    execute_1_controller(instruction_select, fetch_enable, X_output[-1],
                         execute_1, A_select, jump, memory_write, 
                         send_A_to_B, X_write_enable, D_write_enable)

    # All of these are outputs                             
    microcycle_counter(execute_1, execute_2, fetch_1, fetch_2)
    fetch_2 = I_write_enable  # that is, they’re two names for the same wire
    # the last argument is the output
    AND(fetch_enable, execute_2, get_A_from_D)
    OR_6input(get_A_from_D, ..., A_write_enable)
    A_input_mux(A_select, get_A_from_D, 
                I_sign_extended, P_output, X_minus_A, X_nand_A, D, X_output, 

    increment_P = fetch_1
    P_input_controller(jump, increment_P, A_output, B_output, P_input)
    OR(fetch_1, ???, memory_read)

    I_sign_extended = I[-2] || I[:-1]

Most of these pieces are simple multiplexers or registers of one sort or another. The microcycle_counter is just a 4-bit ring counter; the instruction_decoder is just a 6-way decoder of its 4-bit input. However, the execute_1_controller requires a little more clarification.

execute_1_controller(instruction_select, instruction_is_fetch, X_highbit,
                     execute_1, A_select, jump, memory_write, 
                     send_A_to_B, X_write_enable, D_write_enable):
    # The A_select output is 5 bits; it’s used to determine where
    # the A register gets read from if it gets read during the
    # execute_1 microcycle.  We don’t care what value it has the
    # rest of the time.
    A_select = (get_A_from_I_sign_extended, get_A_from_P,
                get_A_from_X_minus_A, get_A_from_X_nand_A,
    # The instruction_select input is also 5 bits, representing
    # the five instructions that can affect A, other than fetch.
    instruction_select = (instruction_is_immediate, instruction_is_jump, 
                          instruction_is_subtract, instruction_is_nand,

    # Here’s how those outputs are computed.                                        
    get_A_from_I_sign_extended = instruction_is_immediate
    NOT(X_highbit, X_not_highbit)
    AND(instruction_is_jump, X_not_highbit, get_A_from_P)
    get_A_from_X_minus_A = instruction_is_subtract
    get_A_from_X_nand_A = instruction_is_nand
    AND(instruction_is_jump, X_highbit, failed_jump)
    OR(failed_jump, instruction_is_store, get_A_from_X)

    jump = get_A_from_P
    AND(execute_1, instruction_is_store, memory_write)

    OR(instruction_is_store, instruction_is_fetch, memory_access)
    AND(execute_1, memory_access, send_A_to_B)

    AND(execute_1, instruction_is_immediate, X_write_enable)

    AND(execute_1, instruction_is_store, D_write_enable)

The ALU simply has to compute X_minus_A and X_nand_A, which constantly feed into the A_input_mux. X_minus_A requires 12 full subtractors (analogous to full adders) and X_nand_A requires 12 NAND gates.

Probable errors:

  • I wasn’t clear who was responsible for setting the write enables on the various registers.
  • Some of the execute_1_controller outputs probably need to be zero when execute_1 is zero.
  • I don’t know anything about memory interfaces and so the memory controller is omitted entirely.

Size Estimate

My initial estimates for number of 2-input NAND gates were:

bit-serial 12-bit parallel
microcycle counter 28 NANDs 28
rest of control 127 578
registers 384 260 (no shifting needed)
subtractor 25 204
NAND 1 12
bit counter 70 0
total 635 1082

I was estimating a 5-gate D latch per bit in the parallel case, or an 8-gate-per-bit master-slave D flip-flop per bit in the serial case. My microcycle counter was going to be a pair of D flip-flops, four AND gates to compute the output bits, and an OR on two of those outputs to compute the new high bit.

I figured on a ripple-carry subtractor that would cost about 17 NAND gates per output bit, although I didn’t actually design one.

Because of the relative paucity of N-bit-wide data paths, going bit-serial doesn’t actually save many gates, but it would slow the machine down by a substantial factor.

Possible Improvements

Dropping the get-X-from-A path on skipped jumps would simplify the processor, probably without making it any harder to use.

A third register (or more) on the stack wouldn’t affect the instruction set at all, but would simplify some code. For example, the code for “add the values stored at memory locations a and b, leaving the sum in the A register” would simplify from:

$0 $a @ - $tmp ! $b @ $tmp @ -


$a @ $0 $b @ - -

Using one-hot encodings of the instructions would require using seven bits of the I register instead of four, but would almost eliminate the instruction decoder. (You'd still need to ensure that the instruction wasn’t $.)

Alternatively, you could pack three, four, or five instructions into each 12-bit word of memory, as Chuck Moore’s c18 core does, instead of one. The "I" instruction, if encodable in lower-order positions, could simply sign-extend more bits, reducing the number of encodable immediate constants in such a case, but not to zero.

You could try a different bit width. 2048 instructions of code is close to a minimum to run, say, a BASIC interpreter.

If N × N → N bit LUTs and registers are available as primitives, the total complexity of the device could drop by an order of magnitude. For example, with 5 × 5 → 5 bit LUTs, you could construct a 4-bit subtractor with borrow-in and borrow-out as a single LUT, and chain three of them together to get a 12-bit subtractor. Even if you have only 4 → 1 bit LUTs, like on a normal FPGA, you can implement a full subtractor in two LUTs attached to the same inputs, instead of 17 (or however many) discrete NAND gates.

If the stack were a little deeper, a “dup”, “swap”, or “over” instruction might help a lot with certain code sequences by reducing the number of immediate constants.

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