General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

instructions LOADIN 1 and LOADIN 2 (and similarly for MOVE S T ).
Note furthermore, that the addition logic in LOADIN j is simply for convenience (most assembler
languages have it, since working with address offsets is commonplace). We could have always
imitated this by a simpler relative load command and an ADD instruction.
A very important ability we have to add to the language is a set of instructions that allow us to
re-use program fragments multiple times. If we look at the instructions we have seen so far, then
we see that they all increment the program counter. As a consequence, program execution is a
linear walk through the program instructions: every instruction is executed exactly once. The
set of problems we can solve with this is extremely limited. Therefore we add a new kind of
instruction. Jump instructions directly manipulate the program counter by adding the argument
to it (note that this partially invalidates the circuit overview slide above 15 , but we will not worry EdNote:15
about this).
Another very important ability is to be able to change the program execution under certain
conditions. In our simple language, we will only make jump instructions conditional (this is
sufficient, since we can always jump the respective instruction sequence that we wanted to make
conditional). For convenience, we give ourselves a set of comparison relations (two would have
sufficed, e.g. = and