Check it out at https://jacobsweeten.github.io/4-Bit-Computer/

Draw Sonic (Sort of):

movi A 0
movi MB 15
stoi A 1
addi A 1
stoi A 1
addi A 1
stoi A 1
addi A 1
stoi A 15
addi A 1
stoi A 15
addi A 1
stoi A 1
addi A 1
stoi A 1
addi A 1
stoi A 1
addi A 1
stoi A 15
addi A 1
stoi A 1
addi A 1
stoi A 12
addi A 1
stoi A 1
addi A 1
stoi A 8
addi A 1
stoi A 15
addi A 1
stoi A 8
addi A 1
stoi A 15

Draw all the colors to the screen:

movi MB 15
movi A 0
movi C 15
sto A A
addi A 1
cmp A C
bneqi 3
sto A A

Call Functions:

# Establish Stack
movi MB 0
movi SB 15
movi SP 15

# Vars
movi A 7

# Get ready to call function
# Push return address
stoi SP 9
subi SP 1
stoi SP 0
subi SP 1

# Branch to function
bi 11

# After return
movi A 9

# Go to end
bi 36

# Function
# Establish stack
sto SP SB
subi SP 1
mov SB SP

# Push A
sto SP A
subi SP 1

# Do stuff
movi A 5

# Return
# Pop A
addi SP 1
load SP A

# Pop SB
addi SP 1
load SP SB

# Get return address
addi SP 1
loadpc2u SP
addi SP 1
load SP B
addpc2 B

# Ret
swappc

• movi: Move immediate into register. Usage: movi [Register] [Value] Analog: A = 1
• mov: Move value in second register into first register. Usage: mov [Register A] [Register B] Analog: A = B
• addi: Add immediate to register and store in register. Sets overflow flag if result is greater than 15. addi [Register] [Value] Analog: A = A + 1
• stoi: Store immediate into memory at address in register. Usage: stoi [Register] [Value] Analog: *A = 1
• sto: Store value in second register into memory at address in first register. Usage: stoi [Register A] [Register B] Analog: *A = B
• subi: Subtrack immediate from register and store in register. Sets underflow flag if result is less than 0. Usage: subi [Register] [Value] Analog: A = A - 1
• muli: Multiply by immediate. Result is stored in HIGH:LOW. Usage: muli [Register] [Value]
• add: Add two registers and store in first register. Sets overflow flag if result is greater than 15. Usage: add [Register A] [Register B] Analog: A = A + B
• sub: Subtract two registers and store in first register. Sets underflow flag if result is less than 0. Usage: sub [Register A] [Register B] Analog: A = A - B
• load: Load value from memory at the address in first register and store in second register. Usage: load [Register A] [Register B] Analog: B = *A
• b: Jump to code at address in register. Usage: b [Register] Analog: goto a
• bi: Jump to code at immediate address. Usage: bi [Value]
• cmp: Compare two registers and set the EQ flag if they are equal. Usage: cmp [Register A] [Register B]
• beqi: Jump to code at immediate address if EQ flag is set. Usage: beqi [Value]
• beq: Jump to code at address in register if EQ flag is set. Usage: beqi [Register]
• bneqi: Jump to code at immediate address if EQ flag is not set. Usage: bneqi [Value]
• bneq: Jump to code at address in register if EQ flag is not set. Usage: bneqi [Register]
• addpc2i: Add immediate to PC2. Usage: addpc2i [Value]
• addpc2: Add value in register to PC2. Usage: addpc2 [Register]
• loadpc2u: Load upper 4 bits of PC2 from address in register. Usage: loadpc2u [Register]
• stopc2u: Store upper 4 bits of PC2 into address in register. Usage: stopc2u [Register]
• stopc2l: Store lower 4 bits of PC2 into address in register. Usage: stopc2l [Register]
• swappc: Move PC2 into PC. Usage: swappc

The computer has 16 memory banks (0-15), selected by the MB register. Each bank holds 16 4-bit words (8 bytes). There is a total of 128 bytes in memory.

The screen uses 4-bit color (RGGB) and the screen buffer uses all of memory bank 15. Offset 0 is the top-left, and it continues from left-to-right, top-to-bottom.