HVM is a simple stack based opcode interpreter. I wrote this in early 2003 over a weekend when I’m sure I should have been doing something else. Perhaps someone will find it useful for something.

It provides the following opcodes

Add And BlkLen Br Brfalse Brtrue Call Ceq Cgt Clt Dbg DecBlk DecInt DecStr Div Dup Exit Func Inc Lbl LdIdx Ldstr Maxstack Mod Mul Neg Not Or Pop PopN Print PrintLn Push Read ReadLn Ret StIdx Sub Xor

These allow for basic flow control (conditionals & function calls) and console i/o.

Here is a basic example of looping from 1 to 10 and printing out the current number:

;
; for(int i = 0; i < 10; i++)
; {
;     print(i)
; }
; print("done")
;

decint @i 0

.lbl for_start
push @i
push 10
clt
brfalse for_done
push @i
inc
println
br for_start

.lbl for_done
ldstr "done"
println

And here’s one that prompts the user, reads an integer from the console and determines if the value is prime or not.

; prime.hvm
;
; prompts the user for a number and prints out whether or not it is prime.

decstr @is " is prime"
decstr @isnot " is not prime"
decint @val 0

ldstr "please enter a number: "
print
readln

; trick to verify the input was an int - or convertable to one
dup
inc
popn 1

; store a copy as our orig value
dup
pop @val

; test if value is one or less
push @val
push 2
clt
brtrue isNotPrime

; test is value is 2 or 3
push @val
push 4
clt brtrue isPrime

; test if even
push @val
push 2
mod
push 0
ceq
brtrue isNotPrime

; for(int i = 0; i < val/2+1; i++) 
;     if val % i == 0 goto isNotPrime
; goto isPrime

decint @i 3
decint @valHalf 0
push @val
push 2
div
inc
pop @valHalf
push @i

.lbl for_block_start
push @valHalf
clt
brfalse isPrime
push @val
push @i
mod
push 0
ceq
brtrue isNotPrime
push @i
inc
br for_block_start

.lbl isPrime
push @is
push @val
print
println
exit

.lbl isNotPrime
push @isnot
push @val
print
println
exit

Pretty straight forward. Not a great example of a parser, opcode design or execution runtime. But it’s a very simple introduction to the three.