SAMDOS

SamDOS 1.1 was the original DOS for the Sam Drive and external drives via the External Drive Interface.

The disk came with the drive and/or interface and also contained disk copies of the tape based utilities supplied with the SAM Coupé.

Versions

Version 1.1 was the source of many problems highlighted in one of the most corking ROM bugs in ROM 1.0 that prevented correct bootstapping forcing the user to enter a CALL command (CALL 229385 for 256K or on a 512K CALL 491529) after pressing F9 or typing BOOT.

Additionally a correction sheet was sent to fix a bug in the HIDE/PROTECT commands (POKE 235973,0 on a 256K and POKE 498117,0 on 512K.

Version 1.3 has been found which correctly boots on ROM 2 and later but does not load the AUTO file

SamDOS 2.0 was released along with ROM 2.0 and booted correctly, although this episode forced MGT to ship new ROMs to all customers.

It contained the updated tape based utilities as well as some bonus BASIC demonstrations.

Information from Frode Tennebø from the Sam Coupé Scrapbook

Introduction

SAMDOS has been designed specifically for the SAM Coupe computer. It is similar and mostly compatible with GDOS & G+DOS as used with the MGT DISCiPLE and Plus D Spectrum disk interfaces.

These formats form the MGT file format family, this is described on the MGT Filesystem page.

Disk Format

We use 80 track per side, giving 160 tracks per disk. A track is made up of 10 data sectors, each giving 512 bytes of storage.

The first 4 tracks of the disk are given up to the SAMDOS directory, leaving 156 tracks available for storage. This leaves available 1560 data sectors of 512 bytes (798720 bytes).

Although each data sector can hold 512 bytes, only 510 bytes of them are available for storage. The last two bytes of the data sector are used by the DOS to locate the next part of the file stored. Byte 511 hold the next track used by the file, while byte 512 holds the next sector.

DOS Booting

In order to boot a disk the DOS boot code must be in the first directory slot (1) on the disk, additionally once booted it will search for the first file starting with the four charaters 'AUTO' (case insensitive) to load and run according to the entries file type.

Disk File Header

At the beginning of each disk file there is a file header. The file header is 9 bytes long:

Details of the Plus D header can be found in the technical information for the Plus D.

File Types

Each file type in the SAMDOS is allocated a numeric identifier:

Modulo Length & Number of Pages

In the SAMDOS header the length of the file is calculated by multiplying the number of pages (byte 7) by 16384 and adding the modulo length (word 1-2), LSB/MSB, ie the length MOD 16K.

Offset Start & Starting Page Number

Read starting page number (byte 8). AND this with 1FH to get the page number in the range 0 to 31. To find the start, multiply the page number by 16384, add the offset and subtract 4000H (since the ROM occupies 0-3FFFH).

When SAMDOS is paged in it resides at 4000H, and ROM0 is placed at 0-3FFFH.

SAMDOS Directory

The first 4 tracks of the disk are allocated to the disk directory, starting at track 0, sector 1. These 4 tracks give us 40 sectors each split into two 256 bytes entries. Each of these entries will identify one file, thus allowing up to 80 entries in the directory.

The format of each directory entry is as follows:

In SAMDOS, unused entries should be set to value 255, MasterDOS shares this but B-DOS uses value 32.

Sector Address Map

SAMDOS allocates 195 bytes to the sector address map, giving 1560 bits, which is the exact number of sectors available for storage on the drive.

A sector address map is calculated for each directory entry. When a field is created a directory entry is made for that file. A sector address map is created by setting the specific bit(s) corresponding to the sector(s) allocated to the file. (Bit 0 of the first byt is allocated to trac4 sector 1). For example, if the file uses 5 sectors then fice corresponding bits in the sector adress map are set and svaed as part of the directory entry.

Bit Address Map (BAM)

The bit address map is not stored on the disk by SAMDOS. It is genereated by performing a bitwise OR of each file’s sector address map. This then gives SAMDOS a usage map of the disk.

When a file is created the first thing SAMDOS does is calculate the BAM and then by looking at the available sectors (ie bits not set), it can work out if there is room for the file. If there is room for the file, then the directory entry is created, including the sector address map specific to the new file, and the file is stored in the sectors which have been specified in the file’s sector address map.

Hook codes