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Page 3-25 of Inside Macintosh: Devices describes the format of a partition map entry. The definition of the pmSig field says:

The partition signature. This field should contain the value of the pMapSIG constant ($504D). An earlier but still supported version uses the value $5453.

It's easy to find information on the $504d ("PM") partitions, which became known as APM. I'm not having much luck finding information on the older $5453 ("TS") partitions.

Most utilities aren't aware of "TS" partitions. Those that do (like libhfs) just check for it so they can present a specific "not handled" error message. The 1985 version of Inside Macintosh doesn't mention it; the disk-related chapters are about MFS filesystems on floppy disks. I didn't spot a link on wikipedia, and google searches haven't turned up anything.

Instance of the "TS" partition format seem to be very rare, but I noticed it on some old Apple CD-ROM images (info here). I can find a chunk with an "LK" signature that appears to be the boot blocks, followed by a block with a "BD" signature that holds the HFS filesystem master block, and a straightforward interpretation of the first several bytes in the "TS" entry seems to match up with the rest of the file contents. Unfortunately there seems to be only one partition on the images I have, so I'm not sure what it would look like if there were more than one.

Is this format documented?

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The first Macintosh that supported partitions was the first to support hard disks: the Macintosh Plus, through its SCSI port. This is documented in chapter 31 "The SCSI Manager" of Inside Macintosh volume IV, 1986, p. IV-292:

For each attached SCSI device, the ROM attempts to read in its driver prior to system startup. In order to be loaded, the device must place two data structures in the first two physical blocks. A driver descriptor map must be put at the start of physical block 0; it identifies the various device drivers available for loading (see Figure 2). The drivers can then be located anywhere else on the device and can be as large as necessary.

Figure 2

A second data structure, the device partition map, must be put at the start of physical block 1; it describes the allocation of blocks on the device for different partitions and/or operating systems (see Figure 3).

Figure 3

Field Size Description
pdSig word always $5453
pdStart long word starting block address
pdSize long word number of blocks
pdFSID long word file system ID ("TFS1" for Macintosh Plus)

It's curious (but allowed on a 68000-based system) that the longwords are not longword-aligned.

Since there's no field in the device partition map for specifying the number of partitions, you need to signal the end of the map with a partition whose pdStart, pdSize, and pdFSID fields are set to 0.

Rather straightforward. There are no frills like partition names or attributes.

I don't know why this partition format later used the "TS" prefix. Perhaps it means "The SCSI", or perhaps it was someone's initials; it might be related to the "TFS1" ID.

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  • I have rolled back an edit that incorrectly turned my own commentary into part of the quote.
    – DrSheldon
    Commented Feb 24, 2023 at 3:07
  • Besides the "TS" I was also wondering about the "ER". Commented Feb 24, 2023 at 7:39
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    @hippietrail: Often they were the initials of internal projects (or of their programmers) that weren't expect to have source code exposed to the public. I don't have specifics on these particular projects; they quickly were obsoleted by better technology, so there aren't as many records of them.
    – DrSheldon
    Commented Feb 25, 2023 at 2:56

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