Legacy Document

Inside Macintosh: Operating System Utilities /

Chapter 9 - Start Manager / Start Manager Reference

Routines / Identifying and Setting the Default Operating System

GetOSDefault

You can use the GetOSDefault procedure to identify the operating system that gets booted on the Macintosh computer.

Procedure GetOSDefault (paramBlock: DefOSPtr);

paramBlock

A pointer to a default operating system parameter block.

<--	sdReserved	byte	Reserved.
<--	sdOSType	byte	Operating-system type.

DESCRIPTION

The GetOSDefault procedure identifies the operating system that gets booted on the Macintosh computer. A value of 1 returned in the sdOSType field indicates the Macintosh Operating System. Apple Computer, Inc. reserves the numbers 0 through 15 for its use.

When the Macintosh Operating System boots, certain startup routines call GetOSDefault and compare the value it returns with the value in the ddType field of the driver's portion of the driver descriptor record. Each driver for the startup device has its own block of fields in this record. The startup routine tries to match the operating-system type returned by GetOSDefault with the value in one of the ddType fields. If it finds a match, the computer continues to boot; if it doesn't, the startup routine searches other drives attached to the computer. The boot process does not continue until the startup routine finds a ddType value that matches the one returned by GetOSDefault.

ASSEMBLY LANGUAGE INFORMATION

The registers on entry and exit for this routine are

Registers on entry
A0	Address of the default operating system parameter block

Registers on exit
A0	Address of the default operating system parameter block

SEE ALSO

For more information about the default operating system parameter block, see "The Default Operating System Parameter Block" beginning on page 9-19.

For information about the driver descriptor record, see the chapter "SCSI Manager" in Inside Macintosh: Devices.

To specify the default operating system, see the description of the SetOSDefault procedure described next.