8-Bit CPU Simulator in Pure Excel Formulas

A fully functional 8-bit CPU simulated entirely in Microsoft Excel / Google Sheets.
Zero VBA. Zero macros. Zero code. Just formulas.

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What Is This?

A working 8-bit CPU built entirely out of spreadsheet formulas. Every register, every flag, every clock cycle is computed by a chain of IFS, SWITCH, INDEX, BITAND, and MIN/MAX formulas. Change a value in RAM and watch all 64 clock cycles recompute instantly.

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Architecture

Component	Implementation
RAM	256-byte memory grid (16×16 cells), fully editable
Registers	A (Accumulator), B (General Purpose), SP (Stack Pointer), PC (Program Counter)
Flags	Z (Zero), C (Carry), N (Negative)
ALU	ADD, SUB, AND, OR, XOR, INC, DEC, NOT — all pure formulas
Control Unit	Fetch → Decode → Execute via IFS formula chain
Execution Trace	64-step table, one row per clock cycle

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Instruction Set (15 Instructions)

Opcode	Mnemonic	Operation
0x00	NOP	No operation
0x01	LDA #n	A = n
0x02	LDB #n	B = n
0x03	STA @a	MEM[a] = A
0x04	ADD	A = A + B
0x05	SUB	A = A - B
0x06	AND	A = A AND B
0x07	OR	A = A OR B
0x08	XOR	A = A XOR B
0x09	INC	A = A + 1
0x0A	DEC	A = A - 1
0x0B	JMP @a	PC = a
0x0C	JZ @a	if Z=1: PC = a
0x0D	JNZ @a	if Z=0: PC = a
0xFF	HLT	Halt execution

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How to Open

Google Sheets (Recommended)

1. Go to sheets.google.com
2. File → Import → Upload the .xlsx file
3. Open the DASHBOARD tab
4. Change the number in cell C4 and watch everything update instantly

Microsoft Excel (Windows)

1. Open the file, click Enable Editing if prompted
2. Open the DASHBOARD tab
3. Change cell C4, press F9 to recalculate

Microsoft Excel (Mac)

1. Open the file, click Enable Editing if prompted
2. Open the DASHBOARD tab
3. Change cell C4, press Cmd = or Formulas → Calculate Now

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Hands-On Walkthrough

Program 1 — Load a number (simplest possible program)

Go to the RAM sheet. Change the first row (0x00) to:

+0	+1	+2	+3	+4	+5
1	42	255	0	0	0

• 1 = opcode for LDA (load into A)
• 42 = the value to load
• 255 = HLT (stop)

Go to DASHBOARD and step through:

• C4 = 0 → Accumulator shows 0x00. Nothing has executed yet.
• C4 = 1 → Accumulator shows 0x2A. That is 42 in hex. LDA just fired.
• C4 = 2 → STATUS shows HALTED. Program complete.

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Program 2 — Add two numbers

Go to RAM sheet. Change row 0x00 to:

+0	+1	+2	+3	+4	+5	+6	+7
1	7	2	5	4	3	32	255

• 1, 7 = LDA #7 — load 7 into A
• 2, 5 = LDB #5 — load 5 into B
• 4 = ADD — A = A + B
• 3, 32 = STA @32 — store result at memory address 32
• 255 = HLT

Go to DASHBOARD and step through:

• C4 = 0 → Accumulator = 0, everything blank
• C4 = 1 → MNEMONIC: LDA #7, Accumulator = 0x07 (7)
• C4 = 2 → MNEMONIC: LDB #5, REG_B = 0x05 (5)
• C4 = 3 → MNEMONIC: ADD, Accumulator = 0x0C (12, that is 7+5)
• C4 = 4 → MNEMONIC: STA @32, TRACKED MEMORY WRITES shows MEM[0x20] = 12
• C4 = 5 → STATUS: HALTED

Result: 7 + 5 = 12, stored in memory address 32.

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Program 3 — Countdown loop

Go to RAM sheet. Change row 0x00 to:

+0	+1	+2	+3	+4	+5
1	5	10	13	2	255

• 1, 5 = LDA #5 — load 5 into A
• 10 = DEC — A = A - 1
• 13, 2 = JNZ @2 — if A is not zero, jump back to address 2 (the DEC instruction)
• 255 = HLT

Go to DASHBOARD and step through:

• C4 = 0 → Accumulator = 0
• C4 = 1 → MNEMONIC: LDA #5, Accumulator = 5
• C4 = 2 → MNEMONIC: DEC, Accumulator = 4
• C4 = 3 → MNEMONIC: JNZ @02, FLAG_Z = 0, PC jumps back to 0x02
• C4 = 4 → MNEMONIC: DEC, Accumulator = 3
• C4 = 5 → MNEMONIC: JNZ @02, PC jumps back again
• keep stepping...
• C4 = 11 → MNEMONIC: DEC, Accumulator = 0, FLAG_Z flips to 1
• C4 = 12 → MNEMONIC: JNZ @02, but Z=1 so jump is skipped, PC moves forward
• C4 = 13 → STATUS: HALTED, Accumulator = 0

This is your first loop. DEC ran 5 times by jumping backwards, and the CPU stopped itself when the Zero flag fired.

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Program 4 — Subtract two numbers

Go to RAM sheet. Change row 0x00 to:

+0	+1	+2	+3	+4	+5
1	9	2	3	5	255

• 1, 9 = LDA #9
• 2, 3 = LDB #3
• 5 = SUB — A = A - B
• 255 = HLT

Go to DASHBOARD and step through:

• C4 = 1 → Accumulator = 9
• C4 = 2 → REG_B = 3
• C4 = 3 → Accumulator = 0x06 (6, that is 9 minus 3)
• C4 = 4 → HALTED

Bonus: Try A=3 and B=9 (subtract bigger from smaller). Watch the Accumulator clamp to 0 and FLAG_C flip to 1 — the carry flag telling you the result went negative.

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Program 5 — Bitwise AND

Go to RAM sheet. Change row 0x00 to:

+0	+1	+2	+3	+4	+5
1	12	2	10	6	255

• 1, 12 = LDA #12 — binary 00001100
• 2, 10 = LDB #10 — binary 00001010
• 6 = AND — A = A AND B
• 255 = HLT

Go to DASHBOARD and step through:

• C4 = 1 → Accumulator = 12 (00001100 in binary)
• C4 = 2 → REG_B = 10 (00001010 in binary)
• C4 = 3 → Accumulator = 0x08 (8 — that is 00001000, only the bits that were 1 in BOTH A and B)
• C4 = 4 → HALTED

Go to the ALU sheet and set A=12, B=10. You can see all 8 operations fire at once and the bit-by-bit breakdown at the bottom showing exactly which bits survived the AND.

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Peek Inside the Formulas

Go to CPU ENGINE sheet. Click cell B4.

You will see the Program Counter formula — the formula that decides where the CPU goes next:

=IFS(
  K3=1,   B3,                    // halted, stay put
  I3=255, B3,                    // HLT, stay put
  I3=11,  J3,                    // JMP, go to address
  I3=12,  IF(F3=1, J3, B3+2),   // JZ, jump if zero flag set
  I3=13,  IF(F3=0, J3, B3+2),   // JNZ, jump if zero flag NOT set
  OR(I3=1,I3=2,I3=3), B3+2,     // 2-byte instructions, skip 2
  TRUE(), B3+1                   // everything else, next byte
)

Now click cell C4.

You will see the ALU formula — the formula that does all arithmetic and logic:

=IFS(
  K3=1,   C3,              // halted, preserve A
  I3=1,   J3,              // LDA, load immediate value
  I3=4,   MIN(255,C3+D3),  // ADD, capped at 255
  I3=5,   MAX(0,  C3-D3),  // SUB, floored at 0
  I3=6,   BITAND(C3,D3),   // AND
  I3=7,   BITOR(C3,D3),    // OR
  I3=8,   BITXOR(C3,D3),   // XOR
  I3=9,   MIN(255,C3+1),   // INC
  I3=10,  MAX(0,  C3-1),   // DEC
  TRUE(), C3               // default, preserve A
)

B4 moves through instructions. C4 transforms data. Every row from 4 to 66 is the same two formulas, each referencing the row above it. That chain of 64 rows is the processor running.

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Sheets Overview

Sheet	Purpose
DASHBOARD	Main control panel — change STEP in C4, watch the CPU
RAM	256-byte memory — edit cells here to load programs
CPU ENGINE	64-step execution trace — every row is one clock cycle
ALU	Interactive arithmetic — change A and B, see all 8 operations live
ISA	Full instruction reference with opcodes and examples
ASSEMBLER	Type mnemonics, get byte values to copy into RAM

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License

MIT — use it, teach with it, build on it.