tion $00.$ As we've indicated, the switch reading the value and storing it in memory location memory $[$operand$].$ The value is then

read into location $09.$

At this point, simulation of the first instruction is completed. All that remains is to prepare

the Simpletron to execute the next instruction. Since the instruction just performed was not a

transfer of control, we need merely increment the instruction$-$counter register as follows:

instructionCounter$++$;

This action completes the simulated execution of the first instruction. The entire process $($i$.$e$.,$ the

instruction$-$execution cycle$)$ begins anew with the fetch of the next instruction to execute.

Now let's consider how the branching instructions $-$ the transfers of control$-$are simulated.

All we need to do is adjust the value in the instruction counter appropriately. Therefore, the

unconditional branch instruction $(40)$ is simulated within the switch as

instructionCounter $=$ operand;

The conditional "branch if accumulator is zero" instruction is simulated as

if $($accumulator $==0)\{$

instructionCounter $=$ operand;

$\}$

At this point, you should implement your Simpletron simulator and run each of the SML

programs you wrote in Exercise $7\mathrm{.}36.$ If you desire, you may embellish SML with additional fea$-$

tures and provide for these features in your simulator.

Your simulator should check for various types of errors. During the program$-$loading phase,

for example, each number the user types into the Simpletron's memory must be in the range $-9999$

to $+9999.$ Your simulator should test that each number entered is in this range and, if not, keep

prompting the user to re$-$enter the number until the user enters a correct number.

During the execution phase, your simulator should check for various serious errors, such as

attempts to divide by zero, attempts to execute invalid operation codes, and accumulator overflows

$($i$.$e$.,$ arithmetic operations resulting in values larger than $+9999$ or smaller than $-9999).$ Such seri$-$

ous errors are called fatal errors. When a fatal error is detected, your simulator should display an

error message, such as

$******$ Attempt to divide by zero $******$

$***$ Simpletron execution abnormally terminated $******$

and should display a full computer dump in the format we discussed previously. This treatment

will help the user locate the error in the program.

$7\mathrm{.}38($Simpletron Simulator Modifications$)$ In Exercise $7\mathrm{.}37,$ you wrote a software simulation of

a computer that executes programs written in Simpletron Machine Language $($SML$).$ In this exer$-$

cise, we propose several modifications and enhancements to the Simpletron Simulator. In the exer$-$

cises of Chapter $21,$ we propose building a compiler that converts programs written in a high$-$level

programming language $($a variation of Basic$)$ to Simpletron Machine Language. Some of the follow$-$

ing modifications and enhancements may be required to execute the programs produced by the

compiler:

a$)$ Extend the Simpletron Simulator's memory to contain $1,000$ memory locations to en$-$

able the Simpletron to handle larger programs.

b$)$ Allow the simulator to perform remainder calculations. This modification requires an

additional SML instruction.

c$)$ Allow the simulator to perform exponentiation calculations. This modification requires

an additional SML instruction.