Image File Format for CP/M computers
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This document contains details of a proposed image file format
for use with the Tatung Einstein and other CP/M-based computers.
The image files have three components, in the following order:

1. Header
2. Directory
3. Images

Images will be complete disc images usually, although other images
are possible, such as RAM or ROM images. Image files are intended
to be used on memory cards, as a convenient way of exchanging data
between CP/M computers and PCs or other systems.

Each component is now described in turn.

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1. Header

The header is at the start of the file and is one or more sectors long.
A sector is defined here as 512 bytes, aligned on a 512-byte boundary.
Normally sector 0 is enough for the header, but it could be larger.

Sector 0 format:

Address		Name		Description			Length
-------		----		-----------			------
0000H-0003H	FILE_ID		File ID 			  4	
0004H		FILE_VER	File Version			  1
0005H-0006H	FILE_END	Last sector of file		  2
0007H-0008H	DIR_START	Start sector of directory	  2
0009H-000AH	IMG_START	Start sector of images	 	  2
000BH-000CH	IMG_SIZE	Sector size of images		  2
000DH-000EH	IMG_COUNT	Number of images 		  2
000FH		IMG_TYPE	Type of images			  1
0010H-001FH	IMG_DESC	Description of images		 16
0020H-002FH	OEM_NAME	OEM name			 16
0030H-003FH	COMP_NAME	Computer name			 16
0040H+				{Type specific data}		{varies}

FILE_ID
 43H, 50H, 2FH, 4DH ("CP/M" without quotes)
 Indicates to CP/M hosts that this is start of valid image file

FILE_VER
 Packed binary decimal format for version x.x
 Defined as 10H for version 1.0
 This byte allows format to change in future if required
 What follows is valid for version 1.0

FILE_END
 This value + 1 gives size of image file in sectors
 Maximum file size is (65535+1)*512 bytes = 32 megabytes

DIR_START
 This value allows header to vary in size
 Directory is described fully later

IMG_START
 Defines start of image 1 and, using IMG_SIZE, 
 start of all other images as there are no gaps between them

IMG_SIZE
 Defines size of images inside image file
 Multiple images within same file all have same size

IMG_COUNT
 Defines total number of images available in file
 Not all of these may be in use
 Provided to avoid host CPU calculation

IMG_TYPE
 01H if image file contains disc images
 Other values > 1 if RAM or ROM images (to be decided)
 This byte determines Type specific data at 0040H+

IMG_DESC
 Text string describing images
 e.g. "200KB floppy    " (without quotes)

OEM_NAME
 Text string for computer manufacturer with space padding
 e.g. "Tatung          " (without quotes)

COMP_NAME
 Text string for computer name with space padding
 e.g. "Einstein        " (without quotes)

Type specific data
 This varies depending on Image type byte at address 0005H
 For Image type=01H (disc image), data defined as follows:

 0040H		DSC_VER		CP/M disc version 	1 byte

 DSC_VER
  Packed binary decimal format for CP/M version x.x
  Possible values are 22 or 31 or 41
  Defined as 22 for Einstein images
  This byte determines following data:

  If 22 then
  0041H-004FH	DPB		Disc parameter block	15 bytes

  If 31 or 41 then
  0041H-0051H	DPB		Disc parameter block	17 bytes

  DPBs for CP/M 3.1 or 4.1 have two extra bytes at the end
  For all versions, block size given by BSH or BLM in DPB,
  at addresses 0043H and 0044H respectively
  All bytes after DPB in sector 0 are unused	

N.B. If image file not yet in use all bytes in sector 0 are zero, 
except FILE_ID, FILE_VER and FILE_END (addresses 0000H-0006H).

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2. Directory

The directory is a list of user-defined names for each image
in numerical order, with 16 bytes reserved for each entry.
Format of directory entry:

 Option 1

 Address	Description			Length
 -------	-----------			------
 0000H-000FH	Name 				16 bytes

or

 Option 2	copied from CP/M 3 & 4 Disc label

 Address	Description			Length
 -------	-----------			------
 0000H		20H 				1 byte
 0001H-000BH	Name				11 bytes
 000CH		01H				1 byte
 000DH-000FH	00H, 00H, 00H			3 bytes

In both cases, byte at 0000H is 0E5H if image not yet in use
First byte of first entry is 0E5H if no images yet in use

Directory space needed is Maximum image number/32 sectors, 
rounded up to next whole sector

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3. Images

Images are in a contiguous block after the directory
Disc images are in logical sector order with no skews or gaps

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Draft Version 1.0.1 by Tony Brewer, September 2010

Changes from 1.0:
Added names for header fields
Change order of fields to make them more logical