img353

0: mar<-pc;rd;    / start of rnain loop

1: pc<-pc+l;rd;    /increment PC, uait for memory

2: ir<-rnbr;if n then goto 0;    / transfer instr. to IR, decode top

bit. NB to get frorn MBR to IR it passes through ALU; n is equivalent to top bit (sign bit).

3: tir<-lshift(ir+ir) ; if n then goto 19; /test bit 14,

IR shifted left TWICE into TIR 4: tir<-lshift(tir) ; if n then goto 11; /test bit 13 5: alu<-tir; if n then goto 9;    /test bit 12

/LODD instruction 6: mar<-ir;rd;

7: rd;    /wait for memory

8: ac<-mbr; goto 0;    /LODD completed

/STOD

9: mar<-ir;mbr<-ac;vr; 10:ur; goto 0;

11 :alu<-tir;if n then goto 15;    /0010=addd or 0011 = subd?

/ADDD

12:mar<-ir;rd;

13:rd;

14:ac<-mbr+ac;goto 0;

/SUBD

15:mar<-ir;rd;

16:ac<-ac+l;rd;

17:a<-inv(mbr) ;

18:ac<-ac+a;goto 0;

/get 2nd. operand f rom memory

/wait for memory

/NB. mbr channeled into alu /see ADDD

/see algorithm for subd belou /complement and put in reg. A temporarily.

19 :tir<-lshift (tir) ;if n then goto 25; / test bit 13

20 :alu<-tir;if n then goto 23;    / test bit 12