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16bitjs's Introduction

16-bit Javascript VM

The project consists of:

  • The definition of an assembly language with 37 instructions
  • An assembler to transform a *.asm file into a binary executable format
  • A small virtual machine which simulates a basic computer architecture: A memory space, stack, and CPU with 4 general purpose registers and a fetch-decode-execute cycle

The virtual machine can run in two modes: run (default) and step. Run mode simply runs the entire program in sucession. Step mode runs the program in a debug environment, pausing before executing each instruction and displaying the entire state of the machine.

Running the VM

Running the assembler

node src/assembler -i {infile.asm} -o {outfile.bin}

Running a program

node src -p {program.bin}

Assembly language

The assembly language consists of 16 distinct instructions which support all the basic features you would expect: Arithmetic, Loading values to and from memory/registers, conditional/non conditional jumps, functions, system calls for reading and and writing stdio etc.

Comments start with a ; character. Labels can be defined by :some_label_name on their own line and then referenced in an instruction like so: LDV16 A, :some_label_name.

Examples

A couple of examples illustrating the language can be found in the asm/ folder.

Instruction set

Real instructions

Instruction Arguments 16 bit representation Description
MOV D, S XXXXXXXXSSDD0000 Move value at source register to destination register
LDV D, V VVVVVVVVVVDD0001 Load a value into destination register.
LDA D, M MMMMMMMMMMDD1110 Load a value from memory into destination register
LDM D, M MMMMMMMMMMDD0011 Load the value in destination register into memory
LDR D, S XXXXXXXXSSDD0010 Load the value from memory pointed at by the source register into the destination register
LDP D, S XXXXXXXXSSDD1111 Load the value in source register into the memory address pointed to by destination register
ATH D, S, O, M, B BBBMOOOOSSDD0100 Perform an arithmetic operation on the source and destination registers. O specifies the operation (listed below) and M is the mode, where 0 = place result in destination register and 1 = place result in source register. If the instruction is right or left shift then B specifies the shifting value
CAL D XXXXXXXXXXDD0101 Call a function in memory pointed at by the destination register
RET XXXXXXXXXXXX0110 Return from function
JLT D, S XXXXXXXXSSDD0111 Jump to memory address pointed at by the source register, if value in the A register is less than value in destination register
PSH S XXXXXXXXSSXX1000 Push the value in source register onto the stack
POP D XXXXXXXXXXDD1001 Pop the stack into the destination register
SYS XXXXXXXXXXXX1010 Perform a system call. This is described below in more detail.
HLT XXXXXXXXXXXX1011 Program halt
JMP M MMMMMMMMMMXX1100 Jump to address in memory. Can only reference memory up to 0x3FF.
JMR S XXXXXXXXSSXX1101 Jump to the address pointed at by the source register

Pseudo Instructions

Pseudo instructions are prepocessed by the assembler and expanded into combinations of the real instructions.

Instruction Arguments Expanded length Description
ADD D, S 1 Add destination to source and store the result in destination
ADDS D, S 1 Add destination to source and store the result in source
SUB D, S 1 Subract destination from source and store the result in destination
SUBS D, S 1 Subract destination from source and store the result in source
MUL D, S 1 Multiply destination with source and store the result in destination
MULS D, S 1 Multiply destination with source and store the result in source
DIV D, S 1 Divide destination by source and store the result in destination
DIVS D, S 1 Divide destination by source and store the result in source
INC D 1 Add one to the destination register
DEC D 1 Subtract one from the destination register
LSF D, A 1 Binary shift left the destination register by amount A (max 7)
LSR D, A 1 Binary shift right the destination register by amount A (max 7)
AND D, S 1 Binary and the destination and source, and store the result in the destination
OR D, S 1 Binary or the destination and source, and store the result in the destination
XOR D, S 1 Binary exclusive-or the destination and source, and store the result in the destination
NOT D 1 Binary not (invert) the destination
LDV16 D, V 6 Load a 16 bit value into destination
SWP D, S 3 Swap the values in the source and destination registers
JGE D, A 4 Jump to address A if value in destination regigster is greater than or equal to the A register. Can potentially mutate all registers except A and destination
JEQ D, A 11 Jump to address A if value in destination regigster is equal to the A register. Can potentially mutate all registers except A and destination
JNE D, A 14 Jump to address A if value in destination regigster is equal to the A register. Can potentially mutate all registers except A and destination

Arithmetic Operation table

Operation Value
Add 0000
Subtract 0001
Multiply 0010
Divide 0011
Left shift 0100
Right shift 0101
And 0111
Or 1000
Xor 1001
Not 1010

System calls

A system call in the VM allows the program to ask resources outside of it's context, such as communication with stdin and stdout. System calls are passed off to the os module and can return their results directly into the CPUs registers.

System call Call code
Write to stdout 0000
Read from stdin buffer 0001
I/O Modes
Output
Mode Description B C D
0 Output register in decimal Destination Mode
1 Output register in binary Destination Mode
2 Output register in hex Destination Mode
3 Output register as a character Destination Mode
4 Output string in memory address pointed to by register Start address

####### Input

Mode Description B C D
0 Read single character value of input into register Destination

Debugger

Running with the step option (node src -p {program.bin} --step), enables the step through debugger, giving a overview of memory, stack and registers as the program executes.

Memory:
0000    08c1 0009 0008 0009 1d01 030b 1c81 030b 1d41 030b 1941 030b 0281 030b 000a 16c1
0010    0009 0008 0009 1d01 030b 1c81 030b 1d41 030b 1941 030b 0281 030b 000a 0081 000b
0020    2781 0009 0008 0281 0291 0009 1141 030b 1c41 030b 1d41 030b 1841 030b 1b01 030b
0030    0801 030b 18c1 030b 1a01 030b 1941 030b 18c1 030b 1ac1 030b 0e81 030b 000a 10d7
0040    11a1 0089 0008 1351 0049 0008 0049 0010 000a 1357 00e1 0089 0008 0009 1981 030b
0050    1841 030b 1b01 030b 1cc1 030b 1941 030b 0281 030b 000a 02c1 0291 0009 1381 030b
0060    1bc1 030b 1d01 030b 0801 030b 1141 030b 1c41 030b 1d41 030b 1841 030b 1b01 030b
0070    0801 030b 18c1 030b 1a01 030b 1941 030b 18c1 030b 1ac1 030b 0e81 030b 000a 20d7
0080    21a1 0089 0008 2351 0049 0008 0049 0010 000a 2357 2421 0089 0008 04a1 0089 0008
0090    0009 1981 030b 1841 030b 1b01 030b 1cc1 030b 1941 030b 0281 030b 000a 000c 0000
00a0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00b0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00c0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00d0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00e0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00f0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0100
Page 1/255

Stack:
0000    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0010    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0020    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0030    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0040    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0050    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0060    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0070    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0080    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
0090    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00a0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00b0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00c0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00d0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00e0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
00f0    0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000

Instruction: (LDV) 0000100011000001
Registers:
A: 0000 B: 0000 C: 0000 D: 0000 IP: 0000        SP: 0000

(s)tep (e)xit (n)ext / (p)revious memory page >>>

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