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<!DOCTYPE html>
<html>
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<title>Simple 8-bit Assembler - Instruction Set Help</title>
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<h4>Introduction</h4>
<p>This simulator provides a simplified assembler syntax (based on <a href="http://www.nasm.us" target="_blank">NASM</a>) and is simulating a x86 like cpu. In depth documentation and introduction to assembler can be found on the following websites:</p>
<ul>
<li><a href="http://en.wikipedia.org/wiki/Assembly_language" target="_blank">Assembly - Wikipedia</a></li>
<li><a href="http://cs.smith.edu/~thiebaut/ArtOfAssembly/artofasm.html" target="_blank">The Art of Assembly Language Programming</a></li>
<li><a href="http://www.nasm.us/xdoc/2.10.09/html/nasmdoc3.html" target="_blank">NASM Language Documentation</a></li>
</ul>
<p>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.</p>
<h4>Syntax</h4>
<p>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.</p>
<pre>label: instruction operands ; Comment</pre>
<p>Valid number formats for constants are:</p>
<pre>
Decimal: 200
Decimal: 200d
Hex: 0xA4
Octal: 0o48
Binary: 101b
</pre>
<p>It is possible to define a number using a character or multiple numbers (see instruction <i>DB</i>) by using a string.</p>
<pre>
Character: 'A'
String: "Hello World!"
</pre>
<p>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.</p>
<pre>
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)
</pre>
<h4>MOV - Copy a value</h4>
<p>Copies a value from <i>src</i> to <i>dest</i>. The MOV instruction is the only one able to directly modify the memory. SP can be used as operand with MOV.</p>
<pre>
MOV reg, reg
MOV reg, address
MOV reg, constant
MOV address, reg
MOV address, constant
</pre>
<h4>DB - Variable</h4>
<p>Defines a variable. A variable can either be a single number, character or a string.</p>
<pre>
DB constant
</pre>
<h4>Math operations</h4>
<b>Addition and Subtraction</b>
<p>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.</p>
<pre>
ADD reg, reg
ADD reg, address
ADD reg, constant
SUB reg, reg
SUB reg, address
SUB reg, constant
</pre>
<b>Increment and Decrement</b>
<p>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.</p>
<pre>
INC reg
DEC reg
</pre>
<b>Multiplication and division</b>
<p>Multiplies or divides the <i>A</i> register with the given value. This operations will modify the carry and zero flag.</p>
<pre>
MUL reg
MUL address
MUL constant
DIV reg
DIV address
DIV constant
</pre>
<b>Logical instructions</b>
<p>The following logical instructions are supported: AND, OR, XOR, NOT. This operations will modify the carry and zero flag.</p>
<pre>
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
</pre>
<b>Shift instructions</b>
<p>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.</p>
<pre>
SHL reg, reg
SHL reg, address
SHL reg, constant
SHR reg, reg
SHR reg, address
SHR reg, constant
</pre>
<h4>CMP - Compare</h4>
<p>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.</p>
<pre>
CMP reg, reg
CMP reg, address
CMP reg, constant
</pre>
<h4>Jumps</h4>
<b>JMP - Unconditional jump</b>
<p>Let the instruction pointer do a unconditional jump to the defined address.</p>
<pre>
JMP address
</pre>
<b>Conditional jumps</b>
<p>Let the instruction pointer do a conditional jump to the defined address. See the table below for the available conditions.</p>
<table class="table table-condensed table-striped">
<thead>
<tr>
<th>Instruction</th>
<th>Description</th>
<th>Condition</th>
<th>Alternatives</th>
</tr>
</thead>
<tbody>
<tr>
<td>JC</td>
<td>Jump if carry</td>
<td>Carry = TRUE</td>
<td>JB, JNAE</td>
</tr>
<tr>
<td>JNC</td>
<td>Jump if no carry</td>
<td>Carry = FALSE</td>
<td>JNB, JAE</td>
</tr>
<tr>
<td>JZ</td>
<td>Jump if zero</td>
<td>Zero = TRUE</td>
<td>JB, JE</td>
</tr>
<tr>
<td>JNZ</td>
<td>Jump if no zero</td>
<td>Zero = FALSE</td>
<td>JNE</td>
</tr>
<tr>
<td>JA</td>
<td>></td>
<td>Carry = FALSE && Zero = FALSE</td>
<td>JNBE</td>
</tr>
<tr>
<td>JNBE</td>
<td>not <=</td>
<td>Carry = FALSE && Zero = FALSE</td>
<td>JA</td>
</tr>
<tr>
<td>JAE</td>
<td>>=</td>
<td>Carry = FALSE</td>
<td>JNC, JNB</td>
</tr>
<tr>
<td>JNB</td>
<td>not <</td>
<td>Carry = FALSE</td>
<td>JNC, JAE</td>
</tr>
<tr>
<td>JB</td>
<td><</td>
<td>Carry = TRUE</td>
<td>JC, JNAE</td>
</tr>
<tr>
<td>JNAE</td>
<td>not >=</td>
<td>Carry = TRUE</td>
<td>JC, JB</td>
</tr>
<tr>
<td>JBE</td>
<td><=</td>
<td>C = TRUE or Z = TRUE</td>
<td>JNA</td>
</tr>
<tr>
<td>JNA</td>
<td>not ></td>
<td>C = TRUE or Z = TRUE</td>
<td>JBE</td>
</tr>
<tr>
<td>JE</td>
<td>=</td>
<td>Z = TRUE</td>
<td>JZ</td>
</tr>
<tr>
<td>JNE</td>
<td>!=</td>
<td>Z = FALSE</td>
<td>JNZ</td>
</tr>
</tbody>
</table>
<b>CALL - Function call</b>
<p>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.</p>
<pre>
CALL address
</pre>
<b>RET - Exit a subroutine</b>
<p>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.</p>
<pre>
RET
</pre>
<h4>Stack instructions</h4>
<b>PUSH - Push to stack</b>
<p>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.</p>
<pre>
PUSH reg
PUSH address
PUSH constant
</pre>
<b>POP - Pop from stack</b>
<p>Pops a value from the stack to a register. This instruction will increase the SP.</p>
<pre>
POP reg
</pre>
<h4>Other instructions</h4>
<b>HLT - Stops the processor.</b>
<p>Stops operation of the processor. Hit Reset button to reset IP before restarting.</p>
<pre>
HLT
</pre>
<h2>Input Switches</h2>
<b>NES buttons inspired switches.</b>
<p>
There are 16 switches you can turn on and off in any moment, those are mapped to the memory as bits on a byte on address 255 for 0-7 and 254 for 8-15.<br />
Those two address are part of the display so you can see they have on the display.
</p>
<hr style="margin-bottom:10px;"/>
<p><small>by Marco Schweighauser (2015) | MIT License | <a href="https://www.mschweighauser.com/make-your-own-assembler-simulator-in-javascript-part1/" target="_blank">Blog</a></small></p>
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