Fork me on GitHub


This simulator provides a simplified assembler syntax (based on NASM) and is simulating a x86 like cpu. In depth documentation and introduction to assembler can be found on the following websites:

The simulator consists of a 8-bit cpu and 256 bytes of memory. All instructions (code) and variables (data) needs to fit inside the memory. For simplicity every instruction (and operand) is 1 byte. Therefore a MOV instruction will use 3 bytes of memory. The simulator provides a console output which is memory mapped from 0xE8 to 0xFF. Memory mapped means that every value written to this memory block is visible on the console.


The syntax is similar as most assemblers are using. Every instruction must be on their own line. Labels are optional and must either start with a letter or a dot (.) and end with a colon.

label: instruction operands	; Comment

Valid number formats for constants are:

Decimal: 200
Decimal: 200d
Hex: 0xA4
Octal: 0o48
Binary: 101b

It is possible to define a number using a character or multiple numbers (see instruction DB) by using a string.

Character: 'A'
String: "Hello World!"

Operands can either be one of the four general purpose registers, stack pointer register, a memory address or a constant. Stack pointer register can only be used as operand in MOV, ADD, SUB, CMP, INC and DEC instructions. Instead of defining an address as a constant or by using a register you can use labels. The assembler will then replace the label with the corresponding constant.

General purpose (GP) register: A, B, C, D
Stack pointer register: SP
Address using a GP register: [A]
Address using a GP register and offset: [D-3]
Address using SP register and offset: [SP+2]
Address using a constant: [100]
Address using a label: label
Constant: Any number between 0..255 (8bit unsigned)
Offset for indirect addressing: Integer between -16..+15 (sign is mandatory)

MOV - Copy a value

Copies a value from src to dest. The MOV instruction is the only one able to directly modify the memory. SP can be used as operand with MOV.

MOV reg, reg
MOV reg, address
MOV reg, constant
MOV address, reg
MOV address, constant

DB - Variable

Defines a variable. A variable can either be a single number, character or a string.

DB constant

Math operations

Addition and Subtraction

Adds two numbers together or subtract one number form another. This operations will modify the carry and zero flag. SP can be used as operand with ADD and SUB.

ADD reg, reg
ADD reg, address
ADD reg, constant
SUB reg, reg
SUB reg, address
SUB reg, constant
Increment and Decrement

Increments or decrements a register by one. This operations will modify the carry and zero flag. SP can be used as operand with INC and DEC.

INC reg
DEC reg
Multiplication and division

Multiplies or divides the A register with the given value. This operations will modify the carry and zero flag.

MUL reg
MUL address
MUL constant
DIV reg
DIV address
DIV constant
Logical instructions

The following logical instructions are supported: AND, OR, XOR, NOT. This operations will modify the carry and zero flag.

AND reg, reg
AND reg, address
AND reg, constant
OR reg, reg
OR reg, address
OR reg, constant
XOR reg, reg
XOR reg, address
XOR reg, constant
NOT reg
Shift instructions

The following shift instructions are supported: SHL/SAL and SHR/SAR. As this simulator only supports unsigned numbers SHR and SAR yield the same result. This operations will modify the carry and zero flag.

SHL reg, reg
SHL reg, address
SHL reg, constant
SHR reg, reg
SHR reg, address
SHR reg, constant

CMP - Compare

Compares two values and sets the zero flag to true if they are equal. SP can be used as operand with CMP. Use this instruction before a conditional jump.

CMP reg, reg
CMP reg, address
CMP reg, constant


JMP - Unconditional jump

Let the instruction pointer do a unconditional jump to the defined address.

JMP address
Conditional jumps

Let the instruction pointer do a conditional jump to the defined address. See the table below for the available conditions.

Instruction Description Condition Alternatives
JC Jump if carry Carry = TRUE JB, JNAE
JNC Jump if no carry Carry = FALSE JNB, JAE
JZ Jump if zero Zero = TRUE JB, JE
JNZ Jump if no zero Zero = FALSE JNE
JA > Carry = FALSE && Zero = FALSE JNBE
JNBE not <= Carry = FALSE && Zero = FALSE JA
JNB not < Carry = FALSE JNC, JAE
JB < Carry = TRUE JC, JNAE
JNAE not >= Carry = TRUE JC, JB
JNA not > C = TRUE or Z = TRUE JBE
CALL - Function call

Call can be used to jump into a subroutine (function). Pushes the instruction address of the next instruction to the stack and jumps to the specified address.

CALL address
RET - Exit a subroutine

Exits a subroutines by popping the return address previously pushed by the CALL instruction. Make sure the SP is balanced before calling RET otherwise the instruction pointer will have an ambiguous value.


Stack instructions

PUSH - Push to stack

Pushes a value to the stack. The stack grows down and the current position is available in the stack pointer register (SP). This instruction will decrease the SP.

PUSH reg
PUSH address
PUSH constant
POP - Pop from stack

Pops a value from the stack to a register. This instruction will increase the SP.

POP reg

Other instructions

HLT - Stops the processor.

Stops operation of the processor. Hit Reset button to reset IP before restarting.


by Marco Schweighauser (2015) | MIT License | Blog