Explain why both the HALT operation code described in Figure 6.5 and the .END ps

Question

Explain why both the HALT operation code described in Figure 6.5 and the .END pseudo-op mentioned at the end of this section are needed in an assembly language program and what might happen if one or both were omitted.

FIGURE 6.5 FIGURE 6.13 Binary Op Code Operation LOAD X STORE X CLEARX ADD X INCREMENT X SUBTRACT X DECREMENT X COMPAREX Instruction Format: Op Code 4 bits Address 12 bits Meaning R ? CON(X) 0-CONÇX Object Program: 0010 Machine Language Instruction 1101 000000001001 1101 000000001010 0000 000000001001 0111 000000001010 1001 000000000111 1110 000000001001 1000 1110 000000001010 ess Meaning 0100 0101 0110 0111 CON(X) + 1 ? CON(X) 0010 LOAD X COMPARE Y JUMPGT DONE OUTX JUMP LOOP OUT Y CON(X)-1-CONCX) if CONCX)> R then GT 1 else 0 if CON(X) = R then EQ = 1 else? if CON(X)

Explanation / Answer

Program Counter contains the address of next instruction to be executed on microprocessor.
A Halt instruction freezes the value of Program counter which leads to no more updation to the value of program counter.Since program counter is not getting updated and its value remains steady,fetching of next instruction is stopped.This will put microprocessor in steady position.microprocessor now in sleep or idle mode,which is accomplished by power down to all logic circuits, which are needed for keeping processor ready for next input instruction.
So,it is good to put microprocessor in halt when there is no more instructions left for execution.

If no Halt instruction given at the end of a assembly program nothing much will happen,but processor will keep on running and Program counter(PC) will keep on fetching next instruction from memory.if there is any other program in memory then it will start executing,or if nothing is stored in memory then PC will fetch garbage value from memory.And processor will start executing these unnecessary instruction cause unpredictive nature of processor with errorneous output.

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