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

sub, similar to add.
mul, and leq need some work.
c○: Michael Kohlhase 291
We will not go into detail for the other arithmetic commands, for example, mul could be implemented
as follows:
label instruction effect comment
〈mul〉 dec IN2 SP: = SP − 1
LOADI 0
STORE 1 P (1): = 0 initialize result
LOADIN 2 1 ACC : = P (SP + 1) read arg 1
STORE 0 P (0): = ACC initialize counter to arg 1
〈loop〉 JUMP= 〈end〉 if counter=0, we are finished
LOADIN 2 0 ACC : = P (SP) read arg 2
ADD 1 ACC : = ACC +P (1) current sum increased by arg 2
STORE 1 P (1): = ACC cache result
LOAD 0
SUBI 1
STORE 0 P (0): = P (0) − 1 decrease counter by 1
JUMP loop repeat addition
〈end〉 LOAD 1 load result
STOREIN 2 0 push it on stack
inc IN1
JUMP 〈jt〉 back to jump table
Note that mul and leq are the only two instruction whose corresponding piece of code is not
of the unit complexity. 16 EdNote:16
For the jump instructions, we do exactly what we would expect, we load the jump distance, add
it to the register IN1, which we use to represent the VM program counter VPC. Incidentally, we
can use the code for jp for the conditional jump cjp.
Control Instructions
label instruction effect comment
〈jp〉 MOVE IN1 ACC ACC : = VPC
STORE 0 P (0): = ACC cache VPC
LOADIN 1 1 ACC : = P (VPC + 1) load i
ADD 0 ACC : = ACC +P (0) compute new VPC value
MOVE ACC IN1 IN1: = ACC update VPC
JUMP 〈jt〉 jump back
〈cjp〉 dec IN2 SP: = SP − 1 update for pop
LOADIN 2 1 ACC : = P (SP + 1) pop value to ACC
JUMP= 〈jp〉 perform jump if ACC = 0
MOVE IN1 ACC otherwise, go on
ADDI 2
MOVE ACC IN1 VPC: = VPC + 2 point to next
JUMP 〈jt〉 jump back
c○: Michael Kohlhase 292
The imperative stack operations use the index register heavily. Note the use of the offset 8 in the
LOADIN , this comes from the layout of VM that uses the bottom eight cells in the data store as a
scratchpad.
16 EdNote: MK: explain this better
163