Now, let's walk through execution of the first SML instruction, $+1009$ in memory location oo$.$

This procedure is called an instruction$-$execution cycle.

The instructionCounter tells us the location of the next instruction to be performed.

fetch the contents of that location from memory by using the Java statement

instructionRegister $=$ memory $[$instructionCounter$]$;

The operation code and the operand are extracted from the instruction register by the statements

operationCode $=$ instructionRegister $/100$;

operand $=$ instructionRegister $\%100$;

Now the Simpletron must determine that the operation code is actually a read $($versus a write, a

load, and so on$).$ A switch differentiates among the $12$ operations of SML$.$ In the switch state.

ment, the behavior of various SML instructions is simulated as shown in Fig. $7\mathrm{.}36.$ We discuss

branch instructions shortly and leave the orhers to you.

rend:

Display the prompt "Enter an integer", then input the integer and store it

in location memory $[$operand$].$

load:

accumulator $=$ memory $[operand]$;

add:

accumulator $+=$ memory$[$operand$]$;

halt: $,$ Terminate the SML program's execution and display the following message:

$$ Simpletron execution terminated

Fig. $7\mathrm{.}36|$ Behavior of several SML instructions in the Simpletron.

When the SML program completes execution, the name and contents of each register as well

as the complete contents of memory should be displayed. Such a printout is often called a com$-$

puter dump $($no$,$ a computer dump is not a place where old computers go$).$ To help you program

your dump method, a sample dump format is shown in Fig. $7\mathrm{.}37.$ A dump after executing a

Simpletron program would show the actual values of instructions and data values at the moment

execution terminated.

REGISTERS :

MEMORY:

$40+0000+0000+0000+0000+0000+0000+0000+0000+0000+0000$

$50+0000+0000+0000+0000+0000+0000+0000+0000+0000+0000$

$70+0000+0000+0000+0000+0000+0000+0000+0000+0000+0000$

$80+0000+0000+0000+0000+0000+000+0000+0000+000+0000+0000$

$90+0000+0000+0000+0000+0000+0000+0000+0000+0000+0000$

$+0000+0000+0000+0000$

Fig. $7\mathrm{.}37|$ A sample dump.