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)
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
Defines a variable. A variable can either be a single number, character or a string.
DB constant
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, constantIncrement 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 regMultiplication 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 constantLogical 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 regShift 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
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
Let the instruction pointer do a unconditional jump to the defined address.
JMP addressConditional 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 |
| JAE | >= | Carry = FALSE | JNC, JNB |
| JNB | not < | Carry = FALSE | JNC, JAE |
| JB | < | Carry = TRUE | JC, JNAE |
| JNAE | not >= | Carry = TRUE | JC, JB |
| JBE | <= | C = TRUE or Z = TRUE | JNA |
| JNA | not > | C = TRUE or Z = TRUE | JBE |
| JE | = | Z = TRUE | JZ |
| JNE | != | Z = FALSE | JNZ |
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 addressRET - 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.
RET
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 constantPOP - Pop from stack
Pops a value from the stack to a register. This instruction will increase the SP.
POP reg
Stops operation of the processor. Hit Reset button to reset IP before restarting.
HLT
by Marco Schweighauser (2015) | MIT License | Blog