Apple Logic Board Informacje Techniczne Pobierz pdf (Strona 36)

E. RESETTING RAMEXPAND ENTRY POINTS

The entry point into RAMEXPAND that is used when calling it

is normally stored in locations 11—12. This entry point corres-

ponds to the first location of the low RAM portion of RAMEXPAND,

which is normally at HIMEM. If locations 11—12 are changed or

clobbered, they must be reset before using RAMEXPAND from BASIC

programs through a CALL 10 or the Applesoft USR function.

F. DECLARING VARIABLES

Variables to be used as parameters in RAMEXPAND command

strings should be declared before use. This can be accomplished

by simply assigning a value to the variable, prior to calling

RAMEXPAND. If this precaution is not taken, RAMEXPAND will

either not return the value expected or flag an error. (see

section IV.A.3.)

IV. ERROR HANDLING

A. DESCRIPTION

Each RAMEXPAND function has an associated error status which

is generally passed to a variable stipulated in the command

string CM$. The following characteristics describe the error

handling process under a variety of conditions.

1. If the command string CM$ is not found, there is no command

string to indicate an error status variable. In this case, there

is no indication of any error.

2. If no variable is specified for the error status, no error

status is returned and the function continues. If no other error

occurs, the function will be successful.

3. If a non—existent variable (i.e., a variable which was not

declared previously) is specified for error status, there is no

indication of any error, even if one occurs. This can be quite

confusing in the event that an error does occur, since no

indication of this error will be given via the error status

variable.

4. If an error status variable is found, but some other variable

is not found, an error is returned. No other action takes place.

5. Actions occuring when other errors take place depend upon the

situation and the error. In most (but not all) cases, no action

will have taken place, after an error condition is encountered.

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Apple Logic Board Informacje Techniczne Strona 36