VeeEm

A litle virtual machine I knocked together in C++. It's completely not useful and right now I'm not even trying to pioneer any new ideas. Just keeping up my system programming skills!

Features

32 32-bit general purpose registers (guaranteed initialized to zero at execution start) and a RISCy instruction set.

Building

Built with Clang on OS X Mavericks

clang++ -std=c++11 -o VeeEm -Iinclude src/*.cpp

Syntax

The VM language is being created more or less off the top of my head and is loosely based on what I recall from RISC architecture, particularly the Texas Instruments MSP430.

Sections

Sections are markers in a source file which are used to reference specific parts of a VeeEm program. Each section in a source file is a period . followed by a distinct name describing the section.

The supported types of sections are defined below. If an unknown section marker is encountered in a source file it is discarded.

.entry

The .entry section tells the machine to begin execution at the instruction immediately following the section label. If the .entry section is omitted then execution begins with the first instruction in the source file.

Example:

MyFunction:
    load r1, $0x03
    return

.entry
call :MyFunction

Labels

Labels are markers in a source file which are used to reference specific instructions. Each label in a source file is a unique alphanumeric name followed by a colon.

LoopHere:

A label's name cannot start with a period . - this is reserved for sections.

A label can be used as a parameter of a jump instruction or to define a function using call.

Instructions

An instruction is one instruction name followed by a space and a sequence of zero or more parameters separated by commas. The parameter list length is dependent on the instruction. Program parsing fails if an instruction is specified with too few or too many parameters.

The following sections describe the syntax of the various types of parameters. Note that not every instruction supports every kind of parameter; for example the conditional jump instructions don't support reading offsets from a register.

Registers

Registers are specified by strings starting with the letter r and followed by a decimal integer 0 through 31.

comp r0, r1
incr r22

Constants

Constants are integers designated by strings prefixed with a '$' and followed by anything that strtol(3) can parse with a base of 0 (decimal, octal, or hexadecimal).

load r0, $0xEEFF
load r1, $-3

Instruction Set

add - Adds together two values and sets the result in a register

Syntax: add dest, src1, src2

Example:
```
load r1, $4
load r2, $3
add r0, r1, r2
```
The add instruction computes src1 + src2 and stores the result in dest. src1 and src2 may be registers or constants. dest must be a register.
call - Set execution to a labeled subroutine

Syntax: call dest

Example:
```
call :Func
...
Func:
    ...
    return
```
dest must be a label. Each call instruction executed should be paired with a corresponding return instruction.
comp - Compares two values and sets a flag in the current execution context

Syntax: comp val1, val2

Example:
```
load r1, $4
load r2, $3
comp r1, r2
```
The comp instruction is used to set internal flags for the next conditional jump instruction. val1 and val2 may be registers or constants.
decr - A synonym for sub dest, dest, 0x01

Syntax: decr dest

Example:
```
load r1, $3
decr r1
```
incr - A synonym for add dest, dest, 0x01

Syntax: incr dest

Example:
```
load r1, $3
incr r1
```
load - Load a value into a register

Syntax: load dest, value

Example:
```
load r1, $0x12345678
load r2, r3
```
value may be a register or a constant. dest must be a register.
pop - Pops a value from the stack into a register

Syntax: pop dest

Example:
```
load r1, $3
push r1
pop r2
```
dest must be a register.

A pop instruction without a corresponding push instruction results in a runtime error.
push - Push a constant value or a value from a register onto the stack

Syntax: push src

Example:
```
load r1, $3
push r1
push $5
```
src may be a register or a constant.
return - Returns execution to an invoking call instruction

Syntax: return

Example:
```
call :Func
...
Func:
    ...
    return
```
return sets the next instruction to be executed to the instruction immediately following the invoking call instruction.

A return instruction without a corresponding call instruction results in a runtime error.
sub - Subtracts one value from another and sets the result in a register

Syntax: sub dest, src1, src2

Example:
```
load r1, $4
load r2, $3
sub r0, r1, r2
```
The sub instruction computes src1 - src2 and stores the result in dest. src1 and src2 may be registers or constants. dest must be a register.
syscall - Invoke a system-defined procedure

Syntax: syscall index

Example:
```
 load r1, $4
 syscall 0
```
index is the zero-based index of the system call to invoke. See System Calls for the list of supported system calls.

Conditional Jumps

Jump to an offset or label position if the last values passed to a comp instruction were:

je - equal
jne - not equal
jlt - val1 less than val2
jgt - val1 greater than val2
jlte - val1 less than or equal to val2
jgte - val1 greater than or equal to val2

Syntax: je offset

Example 1:

load r1, $0x01
comp r1, $0x01
je $-2

Example 2:

  load r1, $0x01
Test:
  incr r1
  comp r1, $0x02
  jne :Test

When using a conditional jump instruction with an offset, the offset is a constant value representing number of instructions from current instruction. In Example 1 the load instruction will be the next instruction executed after the jump.

A label can be used as a the parameter to a jump instruction. When used as a parameter the label is written <colon><labelname> as in Example 2. This is backwards from how labels are specified themselves but it makes parameter type disambiguation a simple step.

Executing a conditional jump instruction without first executing a comp instruction results in a runtime error. Successful execution of a conditional jump clears the comp internal flags so that a subsequent conditional jump will require a new comp to have been performed.

System Calls

DUMPCONTEXT

Call index: 0

Prints a formatted list of the current execution context's registers and flags to the console. This system call is automatically invoked when a runtime error occurs before the system halts.
PUTCHAR

Call index: 1

Pops a value off the stack, interprets the value as a character, and prints it to standard output.

csammis/VeeEm