U1MB and SIDE/2 Feature Sets for the Atari 8-Bits

U1MB and SIDE are tightly integrated when both devices are present, but each is entirely usable on its own:
Ultimate 1MB on its own:

• Three different extended RAM upgrades, up to 1MB
• 4 x OS switcher (user flashable)
• 4 x BASIC ROM switcher (user flashable)
• OS agnostic high-speed SIO driver (providing up to 127Kb/s)
• SIO2BT support for wireless connection to disk servers
• SpartaDOS X disk operating system
• Real-time clock (used by SpartaDOS X for date/time stamps)
• Setup menu with settings stored in battery-backed NVRAM

SIDE on its own:

• SpartaDOS X
• Real-time clock
• XEX loader (also supports BASIC files and certain 8K ROM files) which supports multiple FAT16/FAT32 partitions and read-only CIO access to FAT-hosted files
• Hard disk partitions accessible from SpartaDOS X

U1MB and SIDE together:

• All of the above
• DOS agnostic bootable, CF-based PBI hard disk
• Loader providing support for bootable FAT-hosted ATR disk images, dynamic HDD partition mounting, multi-disk titles (with disk rotation button)

Although the original firmware provides much of the functionality described above, the exhaustive list assumes use of the aftermarket firmware described in this manual:

https://atari8.co.uk...ware-Manual.pdf

The AVG cart already mentioned has a SIDE emulation mode which provides much of the functionality of the SIDE cartridge (both stand-alone and in conjunction with U1MB), although the emulation mode must be explicitly enabled at the start of each session. AVG also lacks the RTC which in any case becomes redundant when the device is used with U1MB. Of course AVG has the advantage of being able to mount banked cartridge ROMs, a feature SIDE lacks (although a forthcoming firmware update includes all four OSS programming carts on the SIDE ROM).

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Any opinions on the RAPIDUS? Haven't seen much coverage. Not really my thing, but it's interesting nonetheless.

Rapidus initially proved rather temperamental on machines already equipped with upgrades - U1MB in particular. Problems ranged from system instability to random unwanted cold reboots on RESET. Many problems were alleviated by Rapidus firmware updates and Lotharek even documents a modification to the EXTSEL signal on the Atari motherboard, intended to address Rapidus stability issues.

You can see the denouement of a four-part build of a 130XE with VBXE, U1MB and Rapidus here:

https://www.youtube....h?v=1lfrrOTnJwM

The owner was satisfied with the machine's functionality, as I recall.
Rapidus may be used as a hard disk accelerator via the 'RAPIDISK' driver supplied with the U1MB firmware. The driver roughly doubles HDD bandwidth by running code from fast linear RAM.

Other interesting uses of the Rapidus include ZX Spectrum emulation.
Regarding VBXE: although VBXE is not overwhelmed by software specifically targeting the device, SpartaDOS X includes a screen driver which allows a VBXE machine to run with a very nice 80 column hardware display. VBXE is also designed to be controlled by U1MB where present; one may enable or disable the 'FX Core' via the U1MB setup menu and change the address at which the hardware is decoded.