Von Neumann architecture VM

My extremely slow Von Neumann VM. Was never designed for perfomance yet it is extremely easy to modify the istruction set.

Assembly

Architecture was inspired by x86, but has little in common except some command names;

Machine is little-endian. Stack grows towards smaller addresses.

General rules

All registers are 4-byte, arithmetics is unsigned only.

Registers:

• General purpose: rax, rbx, rcx
• Stack frame: rsp, rbp
• Utility(no direct access): rip, sf

To access value in register use it's name like

push rax

To access value pointed by register use %

push %rax

Numeric values:

To use numbers you have to show first whether it is a number '$' or an address which machine has to follow '%':

For example to push 1 on stack you:

push $1

But to push value at memory location 1 you:

push %1

Labels

You can use labels at any place in code:

Make a label

.label

refer to a label like to a number (direct or address)

je $hello
jne %bye

Instruction set

Assembly syntax is simmilar to AT&T in terms of order "instruction from to"

All availible instructions are:

• xor – arithmetics below
• oor
• aand
• add
• sub
• mul
• div
• mov - move 4 bytes from one location to another
• movb – moves only 1 byte instead of 4
• nop – No operation
• pop – pop from stack
• push – push item on stack
• call – jump with new stack frame
• ret – return to the calling procedure
• cmp – comparison operator
• je – conditional jumps below
• jg
• jge
• jl
• jle
• jmp
• jne
• syscall – used for IO

Syscalls:

syscall num in rax, params in rbx and rcx:

Syscall nums:

• 0 - exit
• 1 - read(rbx - to, rcx - len) (stdio)
• 2 - write(rbx - from, rcx - len) (stdio)

Sections

Two sections are supported:

• text for code
• bss for data

Entrypoint is always at the begining of the text section

Example code:

Package includes a program in file 'fib.mkasm' which calculates nth fibonacci number for a given n.

Note that code won't compile since comments are not supported.

section .text ; code section

sub $4 rsp ; substract 4 from stack pointer
mov $2 rax ; put 2 in rax (write syscall)
mov $hw rbx ; put string address to rbx
mov %hwsize rcx ; put string len to rcx. Note we have to use % to put the value but not it's address
syscall ; perform the syscall
mov $0 rax ; exit syscall
syscall ; perform the syscall


section .bss ; data section
.hw str "Hello, world!" ; strings identified by str label
.hwsize uint 13 ; uints identified by uint

Compiling code

python3 compiler.py fib.mkasm fib.out

Running compiled binary

python3 machine.py fib.out 65536

where 65536 is machine memory size.

Decompiling already compiled binary:

Featuers:

• Function detection based on calls.

• Label detection based on jumps.

• Data section separation based on usage in code.

  python3 decompiler.py fib.out decompiled.mkasm