Files of this type and auxiliary type contain disk images from Apple's DiskCopy program on the Macintosh.
DiskCopy is a program written by Steve Christensen of Apple Computer, Inc., for internal use in duplicating and distributing 3.5" floppy disks. Because of its utility in distributing disk images on the Macintosh, DiskCopy is used in several Apple developer products even though DiskCopy is not an official Apple product and is not supported as such.
Since the monthly Developer CD Series discs contain many DiskCopy disk images, and since the AppleShare and HFS FSTs in System Software 6.0 and later automatically translate DiskCopy files (HFS file type dImg and creator dCpy) to Apple II file type $E0 and auxiliary type $0005, the format is provided here for your utility use only. Apple does not guarantee that files not generated by DiskCopy will work with DiskCopy.
DiskCopy uses a simple checksum algorithm to help insure data integrity for archived disk images. The algorithm for generating the 32-bit checksum is as follows:
Initialize checksum to zero For each data REVERSE WORD: Add the data REVERSE WORD to the checksum Rotate the 32-bit checksum right one bit (wrapping bit 0 to bit 31)
The following 65816 assembly language routine calculates a DiskCopy checksum. It's not a speedy operation -- it takes about 12 seconds to calculate the checksum on an 800K disk image. Anyone finding an assembly routine that can perform this task in under 5 seconds may apply for their IIgs Certificate of Deityship, as documented in the File Type Note for file type $B6.
(Oh, by the way, any entries have to be under 1K in size--the following routine is 88 bytes. So don't think unwinding loops is your ticket to fame and fortune.)
**************************************************************************** * * Compute checksum for DiskCopy data * * v1.2 by David A. Lyons, 18-May-92 * * MPW IIgs assembly format * * Inputs on stack: * Push pointer to data (long) * Push size of data (long) (Must be even!) * JSL CalcChecksum * STA TheChecksum+2 * STX TheChecksum * * Output: * Checksum in A and X (bytes +0 and +1 in X, bytes +2 and +3 in A) * (The inputs have been removed from the stack) * **************************************************************************** CalcChecksum PROC phd ;save caller's direct page reg lda #0 pha pha ;push initial checksum value (zero) tsc tcd checksum equ 1 oldD equ checksum+4 theRTL equ oldD+2 dataSize equ theRTL+3 dataPtr equ dataSize+4 *** Set dataSize to -(dataSize/2)-1 so we can count up by one *** (instead of down by two) to see when we're done lda <dataSize+2 lsr a eor #$ffff sta <dataSize+2 lda <dataSize ror a eor #$ffff sta <dataSize ldy #0 nextWord inc <dataSize bne moreData inc <dataSize+2 beq noMoreData moreData *** Get next 16-bit word from the data buffer lda [<dataPtr],y xba ;swap to 65816 byte order *** Add the data word to the checksum clc adc <checksum sta <checksum bcc noCksumRoll inc <checksum+2 noCksumRoll *** Rotate the 32-bit checksum right one bit, wrapping bit 0 into bit 31 lda <checksum+2 lsr a ror <checksum bcc bit0was0 ora #$8000 ;if we rotated a 1 out of bit 0, bit0was0 sta <checksum+2 ; then set bit 31 *** Advance to the next word and go back for more iny iny bne nextWord ;go back for more data inc <dataPtr+2 bra nextWord ;go back for next bank of data noMoreData pla xba tay pla xba tax ;pull checksum into YX (put in 68000 order) pld ;restore caller's direct page reg lda 2,s sta 2+8,s lda 1,s sta 1+8,s pla pla pla pla ;discard input values tya rtl EndP END
The following definition is used in this document in addition to those defined for all Apple II file types:
All of the information for a DiskCopy disk image is in the data fork. The resource fork usually contains Macintosh resources (in Macintosh resource fork format), including vers resources listing the checksums. This allows Macintosh users to use the Macintosh Finder's "Get Info..." function to quickly examine the checksums.
Because this is a native Macintosh file format, all the multi-byte constants are stored in Reverse order.
This and all of the other Apple II Technical Notes have been converted to HTML by Aaron Heiss as a public service to the Apple II community, with permission by Apple Computer, Inc. Any and all trademarks, registered and otherwise, are properties of their owners.