mkdisk(1) Make a blank emulated floppy or hard disk for xtrs,


mkdisk -1 filename
mkdisk [-3] filename
mkdisk -k [-s sides] [-d density] [-8] [-i] filename
mkdisk -h [-c cyl] [-s sec] [-g gran] filename
mkdisk {-p|-u} {-1|-3|-k|-h} filename


The mkdisk program is part of the xtrs(1) package. It has two distinct functions: (1) It can make a blank (unformatted) emulated floppy or hard drive in a file. (2) With the -p or -u flag, it can turn the write protect flag on or off for an existing emulated floppy or hard drive file. See the xtrs man page for background information.

The conventional file extensions are .dsk for emulated floppies and .hdv for emulated hard drives, but mkdisk does not enforce this convention; you can use any filename. Other extensions sometimes used for emulated floppies are .jv1, .jv3, .8in, and .dmk.

Making Emulated Floppies

With the -1 flag, mkdisk makes an unformatted emulated floppy of type JV1. 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. 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 5" 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 dir 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 wasted space due to fragmentation but limits the maximum size of the drive.

Seeing that the maximum unpartitioned drive size is less than 13 MB and that the maximum granule size is only 8 KB, 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.

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 at A copy of this html file is also included in the xtrs distribution.

The DMK format is documented in a file on David Keil's web site,; this file is also included with his emulator. Some updates to the 4.00 version of the document: (1) If neither the single density nor 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. This update will be effective in version 4.3 of David's emulator; it is incompatible with previous versions. (2) 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 mkdisk.c):

typedef unsigned char Uchar;
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, currently set 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 
                             [warning: xtrs currently ignores this flag]
                      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); cyl (hard)*/
  Uchar dcyl;      /* 31: Directory cylinder [mkdisk sets to 1; xtrs
                      ignores, but value must be correct if image is
                      to be used with Reed emulators.] */
  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;


mkdisk was written by Timothy Mann (see

The floppy file formats here called JV1 and JV3 were developed by Jeff Vavasour for his MSDOS-based Model I and Model III/4 emulators (respectively). They have become a de facto standard in the TRS-80 emulation community, and much TRS-80 software is available on the Internet in .dsk format. Thanks to Jeff for designing and documenting the formats.

The format here called DMK was developed by David Keil for his MSDOS-based Model 4 emulator. This format has the advantage that it can represent essentially everything the original TRS-80 floppy disk controllers can write, including all forms of copy protected disk. Thanks to David for designing and documenting this format.

The hard drive format was developed by Matthew Reed for his MSDOS-based Model I/III and Model 4 emulators. I have duplicated his format to allow users to exchange .hdv hard drive images between xtrs and Matthew's emulators. Thanks to Matthew for designing the format and providing documentation.