Autopsy:

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from Wikipedia:

The ZX Spectrum +3 looked similar to the +2 but featured a built-in 3-inch floppy disk drive (like the Amstrad CPC 6128) instead of the tape drive, and was in a black case. It was launched in 1987, initially retailed for £249 and then later £199 and was the only Spectrum capable of running the CP/M operating system without additional hardware.

The +3 saw the addition of two more 16 KB ROMs. One was home to the second part of the reorganised 128 ROM and the other hosted the +3′s disk operating system. This was a modified version of Amstrad’s AMSDOS, called +3DOS. These two new 16 KB ROMs and the original two 16 KB ROMs were now physically implemented together as two 32 KB chips. To be able to run CP/M, which requires RAM at the bottom of the address space, the bank-switching was further improved, allowing the ROM to be paged out for another 16 KB of RAM.

Such core changes brought incompatibilities:

• Removal of several lines on the expansion bus edge connector (video, power, and IORQGE); caused many external devices problems; some such as the VTX5000 modem could be used via the “FixIt” device
• Dividing ROMCS into 2 lines, to disable both ROMs
• Reading a non-existent I/O port no longer returned the last attribute; caused some games such as Arkanoid to be unplayable
• Memory timing changes; some of the RAM banks were now contended causing high-speed colour-changing effects to fail
• The keypad scanning routines from the ROM were removed
• move 1 byte address in ROM

Some older 48K, and a few older 128K, games were incompatible with the machine.

The +3 was the final official model of the Spectrum to be manufactured, remaining in production until December 1990. Although still accounting for one third of all home computer sales in the UK at the time, production of the model was ceased by Amstrad at that point.

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