AT&T 3B2/300 Technical Reference Manual page 387

FUNCTIONAL D E S C R I P T I O N - - - - - - - - - - - - - - - - - - - - - - - - - -
Page Registers.
The Page Registers consist of three 12-bit registers. There is one register for each
DMA channel used. (Channel O is not used.) The registers are "write only" and MUST be written as a
half-word quantity, although they are only 12 bits wide. The addresses for the Page Registers are as
follows.
REGISTER
ADDRESS
FLOPPY Ox 45002
UART A Ox 46002
UARTB Ox 47002
The registers contain the upper portion of the address as well as the read/write (Rl WO) bit. The
Rl WO bit indicates the type of memory access. If the Rl WO bit is high [1 ], then the DMAC is going to
read main memory. The Page Register address bits will allow up to 16 megabytes of addressing, but
are limited to 64 kilobytes by the EIO bus. The order of the Page Register bits is as follows.
BIT DATA BIT FUNCTION
00 DOO[l] PA16[1]
01 DOl[l]
PA17[1]
02 D02[1] PA18[1]
03
D03[1] PA19[1]
04 D04[1] PA20[1]
05 D05[1]
PA21[1]
06 D06[1] PA22[1]
07 D07[1]
PA23[1]
08 D08[1] RESERVED
09
D09[1] RESERVED
10 DlO[l] RESERVED
11 Dll[l]
PRlWO
Floppy Control Register.
The floppy control register was implemented to control some of the user
programmable functions. The register is an 8-bit read/write register. The beginning address for the
floppy control register is Ox 40003. The assignments for the floppy control register bits are as follows.
BIT DATA BIT FUNCTION
0
DOO[I] FLOPPY DRIVE SELECT 0
1 DOl[l]
FLOPPY DRIVE SELECT 1
2 D02[1] FLOPPY DRIVE SELECT 2
3
D03[1] FLOPPY DRIVE SELECT 3
4 D04[1]
PRECOMPENSA TION BIT 0
5 D05[1] PRECOMPENSA TION BIT 1
6
D06[1] PRECOMPENSATION BIT 2
7
D07[1] FORCE PRECOMPENSATION
Up to four floppy disk drives can be configured with this system. Bits 0-3 are used to select
which floppy disk drive is used. Bits 4-6 are used by the data separator to determine the amount of
precompensation used when writing the floppy.
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TECHNICAL REFERENCE MANUAL