Apple II
Technical Notes
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                                                  Developer Technical Support

GS/OS
#11:    About EraseDisk and Format

Revised by:    Matt Deatherage                                  November 1990
Written by:    Dave Lyons & Matt Deatherage                         July 1990

This Technical Note explains how an application can tell when a user chooses
Cancel from an EraseDisk or Format dialog box and explains why thefile_sys_ID
field is ignored in class-zero calls.
Changes since July 1990:  Noted that System Software 5.0.3 fixes some of these
anomalies.
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Detecting a Canceled Erase or Format Dialog Box

GS/OS Reference says that EraseDisk and Format return with the carry flag set
and A equal to zero when the user cancels the operation.  This is great, except
that the calls actually return with the carry clear, making a Cancel hard to
distinguish from a successful EraseDisk or Format operation.  This happens in
System Software 5.0.2 and earlier; it works as documented in GS/OS Reference in
System Software 5.0.3 and later.

If you must use 5.0.2 or earlier versions of the system software, this Note
presents a safe way around the problem, which works with all versions of the
System Software:

  1.  In the parameter block for class-one EraseDisk or Format, set the
      fileSysID field to 0.  (See note below.)
  2.  Make the call.
  3.  If the error code is non-zero, there was an error.  Handle it.
  4.  Otherwise, the error code is zero.  Check the fileSysID field in
      the parameter block.  If it is still zero, the user chose to
      cancel the operation.

Note that this method only works for class-one calls. For the class-zero
ERASE_DISK and FORMAT calls, the file_sys_ID word is only an input parameter and
always remains unchanged.


About the Class-Zero file_sys_ID Parameter

Even though fileSysID is an input parameter for the class-zero calls ERASE_DISK
and FORMAT, all versions of the system software ignore thesupplied value and
always give the user a dialog for selecting a file system.  This means no
functionality is lost by putting a zero there.

The reasons for this decision are historical.  Although the Apple IIgs ProDOS 16
Reference indicates that the input parameter file_sys_ID would be used in future
versions to choose destination file systems, ProDOS 16 always returned an error
if the file system specified was not $0001 (ProDOS).

Since this effectively means no ERASE_DISK or FORMAT call can be made under
ProDOS 16 with any file_Sys_ID other than $0001, the GS/OS team chose to ignore
the parameter and always give users the choice when using class zero calls.
Otherwise, no program that existed when GS/OS was released would ever allow
users to choose interleaves or file systems (they would always format for
ProDOS, file system $0001).  (Note that the class-one Format andEraseDisk calls
have a new reqFileSysID parameter; if this field is present, the dialog box is
bypassed.)


Further Reference
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  o  GS/OS Reference
  o  Apple IIgs ProDOS 16 Reference