Atari 800

RESTORE (RES.)
In the description of the FOR/NEXT statement, the stack was defined as a group
of memory addresses reserved for return addresses. The top entry in the stack
controls the number of loops to be executed and the RETURN target line for a
GOSUB.
If a subroutine is not terminated by a RETURN statement, the top
memory location of the stack is still loaded with some numbers. If another
GOSUB is executed, that top location needs to be cleared. To prepare the stack
for a new GOSUB, use a POP to clear the data from the top location in the stack.
The POP command must be used according to the following rules:
1. It must be in the execution path of the program.
2. It must follow the execution of any GOSUB statement that is not brought
back to the main program by a RETURN statement.
The following example demonstirates the use of the POP command with a
GOSUB when the RETURN is not executed:
10 Go~:;us 1000
15 F.:Et-1 LINE 20 WLL t·mT BE E>=:ECUTED
:::1:.1 PF.: I t·n II t·~OPt·1AL F.:ETuft:t·~ FR I t·ns TH I~=; t·1Ef;~:;
AGE. H
30 PF.:WT
11
ABt·iOlmAL PETUF:t-l PPrt-n::; THI::; r·1E
SSAGE.
11
40 POP
999 HiD
1 f1t-:::1~~1 F~~~ I t·iT II t·~o~~J E::<ECiJT I t·i~~ ~:;uE:F.:CHJT I t·1E . !I
1010 GOTO 30
1020 ~:ETUF:ti
Figure 4-11. GOSUB Statement With POP
Format:
Example:
RESTORE [aexp]
100 RESTORE
The Atari Personal Computer System contains an internal "pointer" that
keeps track ofthe DATA statement item to be read next. Used without the op-
tional aexp, the RESTORE statement resets that pointer to the first DATA item
in the program. Used with the optional aexp, the RESTORE statement sets the
pointer to the first DATA item on the line specifed by the value ofthe aexp. This
statement permits repetitive use of the same data.
10 FOR N=1 TO 2
20 READ A
30 RESTORE
40 READ B
50 t·l=A+B
60 PRIHT
11
TOTAL EQUALS
11
;M
70 NE:>\T N
00 END
90 DATA 30,15
Figure 4-12. Restore Program Listing
On the first pass through the loop, A will be 30 and B will be 30 so the total line
50 will print SUM TOTAL EQUALS 60, but on the second pass, A will equal15
Program Statements 21