Table 14 - External 2-to-4 Drive Decode: Drives 0 and 1 Swapped
DRIVE SELECT
OUTPUTS
MOTOR ON
OUTPUTS
DIGITAL OUTPUT REGISTER
(ACTIVE LOW)
(ACTIVE LOW)
nMTR1 nMTR0
Bit 7
X
X
X
1
X
X
X
0
Bit 6
X
Bit 5
X
Bit 4
1
Bit1
0
Bit 0
0
nDS1
nDS0
0
0
1
1
0
0
1
1
1
0
0
1
1
0
0
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
X
1
X
X
X
0
X
1
X
0
1
X
X
1
0
X
X
1
1
X
0
0
0
0
X
0
1
X
X
1
0
X
X
1
1
TAPE DRIVE REGISTER (TDR)
The Tape Drive register (Base Address + 3) is included for 82077 software compatibility and allows the user to assign
tape support to a particular drive during initialization. Any future reference to that drive automatically invokes tape
support. The Tape Select bits TDR.[1:0] determine the tape drive number. Table 15 illustrates the Tape Select bit
encoding. Note that drive 0 is the boot device and cannot be assigned tape support.
The encoding of the TDR depends on the Floppy mode (see section Floppy Modes on page 16). The TDR is
unaffected by a software reset.
Table 15 - Tape Select Bits
TAPE SEL1
(TDR.1)
0
TAPE SEL0
(TDR.0)
0
DRIVE
SELECTED
NONE
0
1
1
1
0
1
1
2
3
Normal Floppy Mode
In Normal mode the TDR contains only bits 0 and 1 (Table 16). During a read in Normal mode TDR bits 2 - 7 are high
impedance. The Tape Select Bits are Read/Write.
Table 16 - TDR Normal Floppy Mode
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
TDR
Tri-state Tri-state Tri-state Tri-state Tri-state Tri-state
Tape
Sel1
Tape
Sel0
Enhanced Floppy Mode 2 (OS2)
The configuration of the TDR in the Enhanced Floppy Mode 2 (OS/2 mode) is shown in Table 17.
Table 17 - TDR Enhanced Floppy Mode 2
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
TDR
Reserved
Drive Type ID
Floppy Boot Drive
Tape
Sel1
Tape
Sel0
Reserved, Bits 6 - 7
Bits 6 and 7 are RESERVED. Reserved bits cannot be written and return 0 when read.
SMSC DS – FDC37N769
Page 23 of 137
Rev. 12/21/2000
SMSC [ SMSC CORPORATION ]
