Making Emulated Floppies

With the -1 flag, mkdisk makes an unformatted emulated floppy of type JV1. Besides -f, no additional flags are accepted.

With the -3 flag (which is the default and should normally be used), mkdisk makes an unformatted emulated floppy of type JV3. Besides -f, no additional flags are accepted.

With the -k flag, mkdisk makes an unformatted emulated floppy of type DMK. With -k, the optional flags -s, -d, -8, and -i can be used to give the emulated floppy special properties. Specifying -s1 limits the floppy to one side; with -s2 (the default), the floppy can be formatted as either one- or two-sided. Specifying -d1 limits the floppy to single-density; with -d2 (the default), the floppy can be formatted in either single- or double-density. Specifying -8 allows the floppy to be formatted in an emulated 8" drive; by default it will work properly only in an emulated 5¼" drive. Setting -s1 or -d1 saves space after the floppy is formatted; setting -8 consumes additional space. Specifying -i activates a peculiar feature in some TRS-80 emulators that causes each formatted sector to appear to be both single- and double-density.

Making Emulated Hard Drives

With the -h flag, mkdisk makes an unformatted emulated hard drive with cyl cylinders, sec sectors, and gran granules (LDOS allocation units) per cylinder. The hard drive will have cylinder number dcyl marked for use as its directory.

You will usually want to use the default values for all these parameters. The default is 202 cylinders, 256 sectors per cylinder (that is, 8 heads and 32 sectors per track), and 8 granules per cylinder. This is the largest hard drive that can be used by all LDOS/LS-DOS operating systems without partitioning the drive or patching the FORMAT command. The details on what nondefault values are possible vary, depending on which of xtrs's two hard drive emulations you are using and which other emulators you want to be compatible with, and it is probably best not to delve into these complexities, but read on if you really want to.

For cyl, the number of cylinders on the drive, the default value is 202, the minimum is 3, and the maximum that can be represented in the HDV file's header is 256. You can use 203 cylinders with LDOS and LS-DOS if you format the drive with Model 4 LS-DOS; a minor bug in Model I/III FORMAT/CMD prevents more than 202 cylinders from being formatted, but the system can use 203 thereafter. 203 cylinders is the absolute maximum for LDOS/LS-DOS drivers that do not support partitioning, including the emulator-specific drivers supplied with xtrs (XTRSHARD/DCT), with Matthew Reed's emulator (HARD/CMD), and with David Keil's emulator (EHARD/DCT).

In xtrs 4.1 and later, and in David Keil's emulator version 6.0 and later, a true emulation of Radio Shack's WD1010-based hard disk controller is also available, which works with the native drivers for the original hardware, such as RSHARDx/DCT and the hard disk drivers for NEWDOS and CP/M. In xtrs, the WD1010 emulation ignores the maximum number of cylinders specified in the HDV file's header and allows the driver to format up to 65536 cylinders. This may be useful if your drivers support partitioning (but why would anyone want to partition an emulated hard drive instead of just making two smaller ones?), or if your operating system supports more than 203 cylinders per partition. Note that although RSHARDx/DCT allows up to 406 cylinders per partition, if you use more than 203, the maximum number of sectors per cylinder is limited to 128, so you gain nothing; the maximum size of a partition is still the same.

For sec, the number of sectors per cylinder, the default value is 256, the maximum is 256, and the minimum is 4. There are some restrictions on the values that will work. For the greatest portability, choose a value that is divisible by 32. With xtrs's XTRSHARD/DCT and David Keil's EHARD/DCT, any value is allowed that can be evenly divided into granules; see the next paragraph. With Matthew Reed's HARD/CMD, if sec is greater than 32, it must be divisible by 32. With the emulation of a real WD1010 in newer versions of xtrs (and probably David Keil's emulator too), sec must always be divisible by 32, because we always emulate a drive with 32 sectors per track and from 1 to 8 heads (tracks per cylinder). The RSHARDx/DCT driver assumes that there are always 32 sectors per track.

For gran, the default value is 8, the maximum is 8, and the minimum is 1. In addition, it is necessary that sec be evenly divisible by gran, and that sec÷gran be less than or equal to 32. This value is used only with the emulator-specific drivers listed above; it is ignored when xtrs is using native hardware drivers such as RSHARDx/DCT.

The maximum size of a hard drive image is controlled by cyl and sec: it can be at most cyl×sec 256-byte sectors. The image file starts out small and grows as you write to more cylinders. The allocation efficiency is controlled by the granule size: LDOS allocates file space in granules. Therefore (1) gran should always be set as large as possible and (2) reducing sec, thereby making the granules smaller, reduces space wasted due to fragmentation but limits the maximum size of the drive.

Seeing that the maximum unpartitioned drive size is less than 13 MiB and that the maximum granule size is only 8 kiB, wasted space should not be much of a concern for most xtrs users. Therefore the default parameters have been chosen to give you the largest drive possible without partitioning.

Embedding the file name in a hard disk image

Matthew Reed's hard drive image file format contains an eight-character field for storing the filename of the hard drive image itself. mkdisk simply uses the given filename as this within-image filename. This works best if filename specifies a short file name in the current working directory. Since modern operating systems permit filenames much longer than eight characters, historically versions of mkdisk would deliberately “spill” the remaining characters of filename into the reserved structure field following the filename (see Technical data, below).

There were two problems with the foregoing approach:

• Modern versions of the standard C library such as glibc will detect the overflow of the destination buffer, emit a lengthy, technical, frightening diagnostic, and abort the program immediately. mkdisk itself offered no explanation.
• mkdisk also did not do any checking to see if the filename string copy might overflow the subsequent buffer, the reserved area.

Now, mkdisk by default writes a truncated (if necessary) copy of filename to the hard disk image file header; a warning diagnostic is emitted in that event.

With the -S option, mkdisk simulates the old behavior, up to the end of the reserved area. In this “spill mode”, a warning diagnostic is emitted if spillage occurs, and the program aborts with an error if the filename is too long to fit in the filename and reserved fields combined.

For maximum portability of your hard drive images, use only ASCII-encoded eight-character filenames in the current working directory. Omit control characters in the range \x00-\x1f and \x7f. These are not “8.3” file names; the “.” character, if used, counts toward the eight-character limit.

Write Protection

With the -p flag, mkdisk turns on write protection for an existing emulated floppy or hard drive. It turns off all Unix write permission bits on the file, and (except for JV1 floppies) also sets a write-protected flag inside the file.

With the -u flag, mkdisk turns off write protection for an existing emulated floppy or hard drive. It turns on Unix write permissions to the file, masked by your current umask and the file's current read permissions. It also clears a write-protected flag inside the file (except on JV1 floppies, which don't have such a flag).

mkdisk currently does not have code to auto-recognize file formats, so the -p or -u flag must be accompanied by either -1 (JV1), -3 (JV3), -k (DMK), or -h (hard disk) to identify the file format. There is also no checking for the correct file format, so if you give the wrong flag, the wrong byte inside your file will be changed.

Technical data

The JV1 format is just an array of 256-byte sectors, in the order (track 0 sector 0, track 0 sector 1, ... track 0 sector 9, track 1 sector 0, ...). It can represent only single-sided, single-density floppies. The directory is assumed to be track 17.

The original JV3 format is documented in the printed manual for Jeff Vavasour's commercial Model III/4 emulator. The xtrs implementation includes some extensions.

Full documentation for both JV1 and JV3 can be found in Common File Formats for Emulated TRS-80 Floppy Disks at Tim Mann's TRS-80 site. A copy of this HTML file is also included in the xtrs distribution.

The DMK format was documented in a file on David Keil's web site, now available via the Internet Archive; this file is also included with his emulator. Some points are worth bearing in mind, particularly if you're attempting to work with copy-protected TRS-80 disks:

• If neither the single-density nor the ignore-density option is set and single-density data is recorded, each single density byte is written twice (i.e., the four bytes 12345678 would be written as 1212343456567878). This ensures that when single- and double-density sectors are mixed, each type occupies the correct relative amount of space in the track.
• Bit 15 of an IDAM offset is 1 if the sector is double-density, 0 if single-density. Bit 14 is reserved; it currently must be 0. The actual offset is in bits 13-0. These offsets are relative to the start of the track header, they must be in ascending order (I hope!!), and an offset of 0 or 0xffff terminates the list.

An HDV (hard disk) image has the following format. This information is based on email from Matthew Reed. There is an initial 256-byte header block, followed by an array of sectors. The geometry of the drive is defined in the header block, which looks like this (from reed.h):

/* Matthew Reed's hard drive format.  Thanks to Matthew for providing


   documentation.  The comments below are copied from his mail


   messages, with some additions. */
/* $Id: reed.h,v 1.2 2008/06/26 04:39:56 mann Exp $ */

typedef struct {


    Uchar id1;         /* 0: Identifier #1: 56H */


    Uchar id2;         /* 1: Identifier #2: CBH */


    Uchar ver;         /* 2: Version of format: 10H = version 1.0 */


    Uchar cksum;       /* 3: Simple checksum:


                             To calculate, add together bytes 0 to 31


                             of header (excepting byte 3), then XOR


                             result with 4CH */


    Uchar blks;        /* 4: Number of 256 byte blocks in header:


                             should be 1 */


    Uchar mb4;         /* 5: Not used, but HDFORMAT sets to 4 */


    Uchar media;       /* 6: Media type: 0 for hard disk */


    Uchar flag1;       /* 7: Flags #1:


                             bit 7: Write protected: 0 for no, 1 for


                                    yes [xtrshard/dct ignores for now]


                             bit 6: Must be 0


                             bit 5 - 0: reserved */


    Uchar flag2;       /* 8: Flags #2: reserved */


    Uchar flag3;       /* 9: Flags #3: reserved */


    Uchar crtr;        /* 10: Created by:


                              14H = HDFORMAT


                              42H = xtrs mkdisk


                              80H = Cervasio xtrshard port to Vavasour


                                    M4 emulator */


    Uchar dfmt;        /* 11: Disk format: 0 = LDOS/LS-DOS */


    Uchar mm;          /* 12: Creation month: mm */


    Uchar dd;          /* 13: Creation day: dd */


    Uchar yy;          /* 14: Creation year: yy (offset from 1900) */


    Uchar res1[12];    /* 15 - 26: reserved */


    Uchar dparm;       /* 27: Disk parameters:


                              (unused with hard drives)


                              bit 7: Density: 0 = double, 1 = single


                              bit 6: Sides: 0 = one side, 1 = 2 sides


                              bit 5: First sector: 0 if sector 0,


                                     1 if sector 1


                              bit 4: DAM convention: 0 if normal


                                     (LDOS), 1 if reversed (TRSDOS 1.3)


                              bit 3 - 0: reserved */


    Uchar cyl;         /* 28: Number of cylinders per disk */


    Uchar sec;         /* 29: Number of sectors per track (floppy);


                              cyl (hard) */


    Uchar gran;        /* 30: Number of granules per track (floppy);


                              gran (hard) */


    Uchar dcyl;        /* 31: Directory cylinder [mkdisk sets to 1;


                              xtrs ignores] */


    char label[32];    /* 32: Volume label: 31 bytes terminated by 0 */


    char filename[8];  /* 64 - 71: 8 characters of filename (without


                                   extension) [Cervasio addition.  xtrs


                                   actually doesn't limit this to 8


                                   chars or strip the extension] */


    Uchar res2[184];   /* 72 - 255: reserved */
} ReedHardHeader;