Question

3. Let's say that Marie added a new command: inc X that adds 1 to the value stored at memory location X. a. Is there an opcode available for this instruction? If so, what opcode would you use? b. Show the RTN for the inc command. Bus 0 Main Memory MAR 2 PC MBR ALU AC InREG 6 OutREG 16-bit bus 4. Write a MARIE program that computes and outputs the absolute value of an input integer.

all info in picture

Answer 1

Ans. a Increment X

org 0

LOAD X /load X into AC

ADD 1 /add 1 to AC

STORE X /store AC in X

Ans b. In this answer instruction fetches data X from memory, after increment it stores it back to memory.

Fetch: MAR<-PC

MBR<-M[MAR] //divides into 2 steps

IR<-MBR

   PC<-PC+1

Decode: Decode IR[15..12] //reversed order of above steps

MAR<-IR[11..0]

Get op: MBR<-M[MAR]

Execute: AC<-MBR+1 //increment X

MBR<-AC // move it back

M[MAR]<-MBR // store it back to x location