vsartor / dtvm

An interpreted bytecode VM writen in C++

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A toy VM implemented in C++

This project is licensed under the terms of the MIT License.
See the LICENSE file for further details.

1. Usage

./dtvm [source] [options...]

Options	Description
-no-acc -no-ansi-color-codes	Tells the application not to use ANSI color codes on outputs.
-parse-and-print	Makes it so the application only parses the code and print the interpreted version to the stdout, not executing it. The only difference between the printed code from the source is that there are no empty lines, labels or comments, and j** instructions hold the index to be jumped to as their arguments.
-rval	Sets the number of registers of the VM to val.
-elabname	Sets the entry point of the program to be at label labname
-show-data	Only takes effect if -parse-and-print was given. Also displays strings with the code.
-debug	Starts the VM into debugging mode

2. Instructions

Instruction	Arguments	Description
halt	None	Halts execution of the VM
noop	None	No operations happen
mov	r1 r2	Copies the value of r1 into r2
push	r1	Push value of r1 into the top of the stack
pop	r1	Pops the top of the stack into r1
inc	r1	Increments the value of r1 by 1.
dec	r1	Decrements the value of r1 by 1.
add¹	r1 r2	Adds value of r1 to r2
sub¹	r1 r2	Subtracts value of r1 from r2
mul¹	r1 r2	Multiplies value of r2 by r1
div¹	r1 r2	Divides r2 by r1
mod²	r1 r2	Modulo operation of r2 by r1
cil	lit r2	Sets the value of r2 to match the integer literal lit
cfl	lit r2	Sets the value of r2 to match the floating point literal lit
ods	lab	Prints the constant string represented by lab.
ofv	r1	Output formatted value of r1 to stdout
onl	None	Print newline and flush stdout
iiv	r1	Reads integer value from stdin into r1.
ifv	r1	Reads floating point value from stdin into r1.
ipf	r1	Prompts failure on reading inputs onto r1. r1 receives =0 for no error and >0 for error.
cmp¹	r1 r2	Compare r1 and r2
cmpz	r1	Compare r1 to the appropriate zero
jmp	lab	Jumps uncoditionally to label lab
jgt	lab	Jumps to label lab last comparison was true for >
jeq	lab	Jumps to label lab last comparison was true for =
jlt	lab	Jumps to label lab last comparison was true for <
call	lab	Jumps to label lab pushing the address of the next instruction into the call stack.
ret	None	Jumps to the address at the top of the callstack and pops it.

¹ Fails if the operands don't have the same type
² Fails if the operands aren't both integers

3. Comments about the assembly

Comments start with ';', and they can either start at the beginning of a line or in the end of a line after a full instruction (the instruction token and the arguments).

Label declarations need to be alone in a line (comments after the declaration aren't counted). A label declaration needs to end with : and not contain any whitespace characters. When referencing the label in a j** instruction, the last : should be removed. Since a halt instruction is always added to the end of the code, a label declared in the end of a file will reference a halt instruction.

The program starts execution on the entry point label, which defaults to _start, but can be set through command line options.

Also, any label .lab starting with a dot is considered a sublabel and is expanded to cl.lab, where cl is the context label, that being the last non-sublabel label read in the file.

Constant strings can be defined using the syntax data string_name "string". They must be defined before any reference to that string happens.

In general, see the examples folder for examples.