AT&T 3B2/300 Technical Reference Manual page 262

- - - - - - - - - - - - - - - - - - - - - - - - - - - FUNCTIONAL DESCRIPTION
3B2 Computer Address Spectrum
Figures 3-3 and 3-4 show the 3B2 computer address spectrums for the Version 2 and Version 3 3B2
computers. As shown in Figure 3-3 and 3-4, the Physical Address (PA) scheme internal to the
3B2 computer uses a 27-bit address scheme (PA26-00). The most significant five bits (PA31-27) are
not used. These unused bits are negated.
All input/output cards and other devices connected to the OMA Subsystem of the system board
[Universal Asynchronous Receiver/Transmitters (UARTS), hard disks, OPORAM size register, floppy
disk, OMA controller] are accessed via the Byte Rotate Unit. The Byte Rotate Unit provides a hardware
mechanism to transfer data to/from a 16-bit data bus from/to a 32-bit data bus while preserving byte
placement within the word. Each feature card slot is assigned a unique 2-megabyte address range.
Address space (2 megabytes per card) is reserved for unimplemented feature card slots 13, 14, and 15.
FUNCTIONAL DESCRIPTION 3-9