Atari 800 Basic Reference Manual

GOSUB (GOS.)
RETURN (RET.)
16 Program Statements
In Figure 4-1, the outer loop will complete three passes (X = 1 to 3 ). However,
before this first loop reaches its NEXT X statement, the program gives control to
the inner loop. Note that the NEXT statement for the inner loop must precede
the NEXT statement for the outer loop. In the example, the inner loop's number
of passes is determined by the STEP statement (STEP Z). In this case, Z has
been defined as 0, then redefined as Z + 2. Using this data, the computer must
complete three passes through the inner loop before returning to the outer loop.
The aexp3 in the step statement could also have been defined as the numerical
value 2.
The program run is illustrated in Figure 4-2.
OUTEF.: LOOP
I t·~t·-IEF.: LOOF'
I t-~t-~EF.: LOOP
I t-~HEP LOOP
OUTEP LOOF'
I t·~t-lE~: LOOP
I t-~t-~EP LOOP
I t-~t-~ER LOOP
OUTEP LOOP
IHt·4ER LOOP
I t·4t-~EF.~ LlJOP
I t·~HE~~ LOOP
Figure 4-2. Nested Loop Execution
The return address for the loops are placed in a special group of memory ad-
dresses referred to as a stack. The information is "pushed" on the stack and
when used, the information is "popped" off the stack (see POP.)
Format: GOSUB lineno
lineno
RETURN
Example: 100 GOSUB 2000
2000 PRINT "SUBROUTINE"
2010 RETURN
A
subroutine* is a program or routine used to compute a certain value, etc. It is
generally used when an operation must be replaced several times within a pro-
gram sequence using the same or different values. This command allows the
user to "call" the subroutine!, if necessary. The last line ofthe subroutine must
contain a RETURN statement. The RETURN statement goes back to the physical
line following the GOSUB statement.
Like the preceding FOR/NEXT command, the GOSUB/RETURN command
uses a stack for its return address. If the subroutine is not allowed to complete
normally; e.g., a GOTO lineno before a RETURN, the GOSUB address must be
"popped" off the stack (see POP) or it could cause future errors.
• Generally, a subroutine can do anything that can be done in a program. It is used to save memory
and program-entering time, and to make programs easier to read and debug.

Atari 800 Specifications

General

Brand	Atari
Model	800
Category	Desktop
Language	English