Two types of memory are available: A stack, and an endless tape. Both hold unsigned 8-bit integers.

Execution starts at the commit pointed to by the master branch. Commit messages can contain a series of single-word instructions, seperated with spaces.

If a commit has only one parent, execution will continue there after executing the current commit.
If a commit has multiple parents (numbered 1, 2, 3, ...), the top stack element will be popped. If that element is n, to go (n-1)-th parent, or to the last one, if there are less than (n-1) parents.
After executing a commit with a tag, jump to the branch with the same name.

Instructions:

getchar: read char from STDIN and place it on the stack
putchar: pop top stack element and write it to STDOUT as a char
getint: read int from STDIN and place it on the stack
putint: pop top stack element and write it to STDOUT as a number
<Number>: push the number on the stack
<Letter>: push ASCII value of that letter on the stack

read: place value of current cell on the stack
write: pop top stack element and write it to the current cell
left: pop top stack value, move left for that many places
right: pop top stack value, move right for that many places

add: pop two topmost stack values, add them, push result on the stack
sub: pop two topmost stack values, subtract top one from bottom one, push result on the stack
dup: duplicates topmost stack value
cmp: pops two top values, pushes 1 if bottommost one is larger, 0 otherwise.