How does the Spectrum +3 know whether a disk is bootable or not?

Question

I've been looking through documentation on The Sinclair/Amstrad Spectrum +3 which was the only Spectrum that came with a built-in floppy disk drive. The disk format was based on The Amstrad CPC and PCW disk format, which was based on CP/M.

I'm struggling with some details of the boot process.

I know there's an optional boot block on side 0, track 0, sector 1, at offset $0010, right after the 16-byte 'disk specification'.

But since this block is optional, there must be a way for the system to detect whether it's present or not.

I know that in some disks the file directory will be here instead, and starting at offset $0000 without a disk specification.

Some disk images seem to be of protected disks with purposely confusing sector layouts that must've made it harder to pirate them back in the '80s, which makes it challenging to figure out by looking at the raw data.

So is there somewhere on the disk layout that I'm missing that indicates to the OS that a boot sector is there or not? Or could it possibly all hinge on that one-byte checksum which seems like it could easily give false positives? If so, is the checksum just a sum of the bytes of the boot sector modulo 256?

If it makes a difference I'm using DSK and EDSK format disk images.

Answer 1

I think some of the answers here are combining determining the disk format with detecting if it's bootable. I've answered how the format is detected on its own question, so here I'll concentrate on the boot process.

The +3 tries to boot from a disc by using the DOS_BOOT function. This selects drive A:, closes any open files on it, and then tries to read logical sector 0 into the +3DOS internal cache. Assuming that worked, it sums all 512 bytes of that sector modulo 256. If the result is 3, it copies the sector to 0FE00h in bank 3, sets the memory paging to the all-RAM 4-5-6-3 configuration, and jumps to 0FE10h.

If the checksum is not 3, DOS_BOOT returns error 35, 'Disk is not bootable'. The loader then falls back to executing the BASIC command LOAD "DISK". If that in turn fails, it selects cassette tape and executes LOAD "" to run the first program on the tape.

There is no requirement that the boot disk should be in the "standard" +3 format, begin with the 10-byte disk specification, or even have reserved tracks - as long as the checksum of the first sector is 3, DOS_BOOT will happily boot from a disk in CPC System or CPC Data format. In this respect, the +3 boot process is more flexible than the otherwise-similar PCW, on which the boot sector has to be physically numbered 1.

Answer 2

From my answer to How do I know where the file directory is stored on a Spectrum +3 disk layout?

Part 26 of the +3 manual is what you need to read. There is even a complete example of how to write boot code and how to store it into the disk. This part is the relevant one regarding which values the disk specification block must have:

bootstart: ; ;The bootstrap sector contains the 16 bytes disk
specification at the start. ;The following values are for a AMSTRAD
PCW range CF2/Spectrum +3 format disk. ;

               db   0                   ;+3 format
               db   0                   ;single sided
               db   40                  ;40 tracks per side
               db   9                   ;9 sectors per track

               db   2                   ;log2(512)-7 = sector size
               db   1                   ;1 reserved track
               db   3                   ;blocks
               db   2                   ;2 directory blocks

               db   02Ah                ;gap length (r/w)
               db   052h                ;page length (format)
               ds   5,0                 ;5 reserved bytes

cksum:         db   0         ;checksum must = 3 mod 256 for the sector

A non bootable disk will have 0 reserved tracks.

Answer 3

On a CPC system, you can determine if the disc is in System format (i.e. bootable) by the number of the first physical sector. If the sector is 41h, the disc is in System format, with 2 reserved boot sectors, whereas C1h means Data format, no reserved sectors.

The PCW and Spectrum +3 can also determine the bootability of a disc from a 16-byte record on track 0, head 0, physical sector 1. The checksum of this record indicates if the disc is bootable:

• 1 - sector contains a PCW9512 bootstrap
• 3 - sector contains a Spectrum +3 bootstrap
• 255 - sector contains a PCW8256 bootstrap

The rest of the sector contains the bootstrap code. Information gleaned from https://www.seasip.info/Cpm/amsform.html

Answer 4

But since this [boot] block is optional, there must be a way for the system to detect whether it's present or not.

There isn't. The +3 (like the Amstrad CPC) doesn't autoboot. Using the Loader on a +3 (or |CPM on an Amstrad CPC) on a disk that's in Data or Vendor format (that is, does not have CP/M or custom boot code in the reserved tracks already mentioned) results in:

after the system has failed to load a valid boot block and reserved tracks containing boot code.

(Data disks have no reserved tracks. System disks have reserved tracks containing boot data and a valid boot block. Vendor disks have reserved tracks, but they're blank. They were meant for software vendors to distribute programs without including proprietary CP/M code. You didn't encounter them very often.)