LMC Questions

1. Describe what this code does: (don't just walk through the code, describe what the code does - you could get a question like this in the exam and a lot of students get it wrong by just walking through the code line by line).

   >        INP
           STA times
   loop    LDA times
           SUB count
           BRZ end
           LDA count
           ADD one
           STA count
           OUT times
           BRA loop
   end     HLT
   count   DAT 0
   times   DAT
   one     DAT 1
   

   This code does not work. OUT times is an invalid instruction. Assuming the following code is meant:

   >        INP
           STA times
   loop    LDA times
           SUB count
           BRZ end
           LDA count
           ADD one
           STA count
           LDA times
           OUT
           BRA loop
   end     HLT
   count   DAT 0
   times   DAT
   one     DAT 1
   

   Then the purpose of the code is to input a number, then print that number that many times, or none if the number is negative.

2. Rewrite the code so that the program does exactly the same, but this time using only BRP, and not BRZ or BRA.

   >        INP
           STA 0
           SUB 5
           SUB 14
           BRP 6 // check that times - count > 0
           HLT   // the check is performed first to
           LDA 0 // guarantee identical behaviour
           OUT   // for non-positive inputs
           LDA 5
           ADD 14
           STA 5
           LDA 0
           BRP 2
           HLT
           DAT 1