LMC Questions

A program written in the Little Man Computer instruction set is given below.
```
>        INP
        STA     num
loop    LDA     total
        ADD     num
        STA     total
        LDA     count
        ADD     one
        STA     count
        SUB     num
        BRZ     end
        BRA     loop
end     LDA     total
        OUT
        HLT
one     DAT     1
num     DAT     0
count   DAT     0
total   DAT     0
```
1. State what the program outputs are for the following inputs.
  
  Input Output
  
  1 1
  
  2 4
  
  3 9
2. State what the purpose of the program is.
  
  To square its input.
3. Explain which registers are used and their values when the line STA count is executed and the accumulator is holding the value 9. The label count refers to memory location 16.
  
  The register in use is the Accumulator, which holds the value 9. Also in use is mailbox¹ 16, the value of which is overwritten by the value held in the accumulator (9) during the execution of the statement.
  
  ¹ 'Mailbox' is the preferred term for locations in memory when using LMC.
4. Whilst the line STA count is being executed, the CPU receives a signal from another process, requiring its attention.
  
  State the name for the signal received by the CPU.
  
  Interrupt
5. The code uses direct addressing. Describe one other mode of addressing.
  
  One other mode of addressing is indirect addressing, wherein the value desired is at a location specified by the location specified in the instruction (this is typically known as a pointer).
  
  For example, if the instruction being processed is JMP ($2A), then first the value in memory at offset 0x2A is read, then the result is used as the jump target; if the value at 0x2A is 0x45 then the JMP will be executed as if it were JMP $45.

Input	Output
1	1
2	4
3	9