AT&T 3B2/300 Technical Reference Manual page 383

FUNCTIONAL D E S C R I P T I O N - - - - - - - - - - - - - - - - - - - - - - - - -
Data Byte Rotate Unit.
The Data Byte Rotate Unit is a collection of buffers used to multiplex the
32-bit data bus to/from the main memory to an 8- or 16-bit data bus for the 1/0 bus. This
accommodates the input/output cards and system board devices in the OMA Subsystem that do not
have a 32 bit capability. The byte rotate unit provides data alignment and packing for 8- and 16-bit
peripherals when they access the 32-bit main memory and for system board CPU when the CPU
communicates directly with the input/output (feature) cards. The system board devices that use the
Data Byte Rotate Unit are listed below:
• Direct Memory Access Controller
• Dual Universal Asynchronous Receiver/Transmitter
• Integral Floppy Disk Controller
• Integral Hard Disk Controller.
The Byte Rotate Unit is controlled by the Sequencer logic. The PSIZE16[0] determines whether a
peripheral is capable of transferring 8 or 16 bits at a time. Address bits 01 and 00 are used to select the
data bus bytes. PSIZE16 is a O for 16-bit transfers; PSIZE16 is a 1 for 8-bit transfers. The PSIZE16[0]
signal is sent by the peripheral (feature card) in response to a chip select signal. When a peripheral is a
16-bit device, 8-bit accesses are done by asserting the appropriate data strobe (PDSl-0[0]). Devices
having 8-bit data interfaces require four passes to transfer a complete 32-bit word. Devices having
16-bit data interface require two passes to transfer a complete 32-bit word.
Byte O (bits 31-24) is the least significant byte. Byte 1 is bits 23-16. Byte 2 is bits 15-08.
Byte 3 (bits 07-00) is the most significant byte. Address bits 01 and 00 are decoded (binary) to select
the data bus bytes 3-0 as applicable. For 8-bit operations both address bits 01 and 00 are used to
select the data bus bytes. For 16-bit operations only address bit 01 is used to select the data bus bytes
(address bit 00 is not used in 16-bit operations). For 16-bit operations either data bus bytes 1 and O or
bytes 3 and 2 are selected by only address bit 01. When address bit 01 is high [1 ], data bus bytes 2 and
3 are selected (bits 15-00). When address bit 01 is negated, data bus bytes O and 1 are selected
(bits 31-16). The data strobes (PDSO[O] and PDSl[O]) are used to select which byte or bytes to access
within the main memory. Figure 3-37 summarizes the decoding of the PSIZE16, PPAOl, PPAOO, PDSO,
and PDSl for the selection of data bytes for the 16-bit peripheral input/output bus and for the 32-bit
main memory accesses.
3-130 TECHNICAL REFEMNCE MANUAL