Section 9: Branching and Looping 109

We’ll key the income into the display before running the program so that it will be in the X-register when the :0 instruction in program line 01 is executed. This instruction will place the test value 20,000 in the X-register and (as explained in Appendix A) move the income into the Y-register. The ~ instruction in program line 02 will exchange the numbers in the X- and Y-registers (as also explained in Appendix A): that is, it will place the income back into the X-register and place the test value into the Y-register. This is necessary because when either the :2 instruction in line 05 or the :1 instruction in line 07 is executed, the number in the X-register is moved into the Y-register; if the ~instruction were not included, the test value 20,000, rather than the income, would be in the Y-register when the

binstruction in line 08 is executed.

Keystrokes fs

fCLEARÎ :0

~

go

Display

 

 

 

07- 43, 33

02

Sets calculator to Program mode.

 

 

 

(Display shows program line at

 

 

 

which execution was halted at end

00-

 

 

of preceding example.)

 

 

Clears program memory.

01-

45

0

Recalls test value into X-register and

 

 

 

places income in Y-register.

02-

 

34

Places income in X-register and test

 

 

 

value in Y-register.

03-

43

34

Tests whether number in X-register

 

 

 

(income) is less than or equal to

 

 

 

number in Y-register (20,000).

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