FIRST Robotic Competition - Program Execution

Lets store that program at location 100 and execute it. Note that the opcodes I show would actually be numbers. For simplicity I will show them as nmemonics instead. Our program in main memory looks like this:

           MVI 1,R0		; prod = 1
           MVI 2,R2		; index=2
    Loop1:
           ADD R0,R2		; prod + index
           MOV ACC,R0		; prod =
           DCR R2		; index-1
           MOV ACC,R2		; index =
           JNZ Loop1		; for()

Address	Data
100	MVI R0
101	1
102	MVI R2
103	2
104 [Loop1]	ADD R0,R2
105	MOV ACC,R0
106	DCR R2
107	MOV ACC,R2
108	JNZ
109	104 [Loop1]

To execute this code we start with the PC set to 100.

Start
- PC=100
  The PC is copied to the MA.
- MA=PC=100
  The RAM location 100 is copied to the MDI.
- MDI=MVI R0; PC=PC+1=101
  The MDI is copied to the IR.
100: MVI 1,R0
- IR=MVI R0
  The instruction is decoded. The PC is copied to the MA.
- MA=PC=101
  The RAM location 101 is copied to the MDI.
- MDI=1
  The MDI is copied to R0.
- R0=MDI=1; PC=PC+1=102
- MA=PC=102
  The RAM location 102 is copied to the MDI.
- MDI=MVI R0; PC=PC+1=103
  The MDI is copied to the IR.
102: MVI 2,R2
- IR=MVI R2
  The instruction is decoded. The PC is copied to the MA.
- MA=PC=103
  The RAM location 102 is copied to the MDI.
- MDI=2
  The MDI is copied to R0.
- R2=MDI=2; PC=PC+1=104
- MA=PC=104
  The RAM location 104 is copied to the MDI.
- MDI=ADD R0,R2; PC=PC+1=105
  The MDI is copied to the IR.
104: ADD R0,R2
- IR=ADD R0,R2
- ACC=R0+R2=3
- MA=PC=105
  The RAM location 105 is copied to the MDI.
- MDI=MOV ACC,R0; PC=PC+1=106
  The MDI is copied to the IR.
105: MOV ACC,R0
- IR=MOV ACC,R0
- R0=ACC=3
- MA=PC=106
  The RAM location 100 is copied to the MDI.
- MDI=DCR R2; PC=PC+1=107
  The MDI is copied to the IR.
106: DCR R2
- IR=DCR R2
- ACC=R2-1=1
- MA=PC=107
  The RAM location 107 is copied to the MDI.
- MDI=MOV ACC,R2; PC=PC+1=108
  The MDI is copied to the IR.
107: MOV ACC,R2
- IR=MOV ACC,R2
  The instruction is decoded.
- R2=ACC=1
- MA=PC=108
  The RAM location 108 is copied to the MDI.
- MDI=JNZ; PC=PC+1=109
  The MDI is copied to the IR.
108: JNZ Loop1
- IR=JNZ
  The instruction is decoded.
- MA=PC=109
  Since ACC is not zero, we jump to Loop1.
- MDI=Loop1=104
- PC=MDI=104
- MA=104
- MDI=ADD R0,R2; PC=PC+1=105
104: ADD R0,R2
- IR=ADD R0,R2
- ACC=R0+R2=4
- MA=PC=105
  The RAM location 105 is copied to the MDI.
- MDI=MOV ACC,R0; PC=PC+1=106
  The MDI is copied to the IR.
105: MOV ACC,R0
- IR=MOV ACC,R0
- R0=ACC=4
- MA=PC=106
  The RAM location 100 is copied to the MDI.
- MDI=DCR R2; PC=PC+1=107
  The MDI is copied to the IR.
106: DCR R2
- IR=DCR R2
- ACC=R2-1=0
- MA=PC=107
  The RAM location 107 is copied to the MDI.
- MDI=MOV ACC,R2; PC=PC+1=108
  The MDI is copied to the IR.
107: MOV ACC,R2
- IR=MOV ACC,R2
  The instruction is decoded.
- R2=ACC=0
- MA=PC=108
  The RAM location 108 is copied to the MDI.
- MDI=JNZ; PC=PC+1=109
  The MDI is copied to the IR.
108: JNZ Loop1
- IR=JNZ
  The instruction is decoded.
- MA=PC=109
  Since ACC is zero, we don't jump.
- PC=PC+1=10A
- MA=10A
- MDI=next instruction; PC=PC+1=10B

The result of this loop, prod, is stored in R0. It was computed the second time thru location 105. That value is 4.

Last modified 11 Dec 2006
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