Description: ProDOS 8: Finding Volume Size using Assembly Language
Header: ProDOS 8: Finding Volume Size using Assembly Language


 This article last reviewed: 8 March 1989

 You can determine the number of blocks on a ProDOS-compatible device from
 assembly language.

 On a ProDOS volume, the total number of blocks available is stored on block
 2, bytes $29 and $2A. The number is in low byte/high byte format: for
 example, an 800K floppy will have $40 at byte $29 and $06 at byte $2A.

 Byte $2A contains the first two digits and byte $29 contains the last two
 digits. $0640 is equal to 1600 decimal, which is the total number of blocks
 on an 800K ProDOS disk.

 If you need to determine the number of blocks on a device from the
 hardware, that information can be obtained regardless of whether or not
 block 2 is intact.

 Check memory locations $Cs01, $Cs03, $Cs05 (s = slot number) for $20, $00,
 and $03, respectively. If these values are found, then the card in the slot
 is a disk controller.

 Check memory location $CsFF. If the value is $00 or $FF, assume that an
 Apple Disk II disk controller card is installed ($00 is 16-sector, $FF is
 13-sector).

 Otherwise, check memory location $Cs07.  If the value does not equal $00,
 the device is not a SmartPort device, and the total number of blocks for
 the drive are stored at memory locations $CsFC and $CsFD in
 low-byte/high-byte order.

 If the total number of blocks is 0, contact the card manufacturer for
 details on how to determine the number of blocks.

 (NOTE: We do not know of any disk controller that does not store the total
 number of blocks at memory locations $CsFC and $CsFD.)

 If the value at memory location $Cs07 is $00, the drive is a SmartPort
 device. To find the number of blocks on the drive, you must make SmartPort
 status call $03 (get DIB). This call is documented in the "Apple IIGS
 Firmware Reference Manual" (Apple # 030-3121-A) on pages 114 through 125,
 with the most specific information starting on page 121.
 Copyright 1989 Apple Computer, Inc.
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