the device into 32-bit I/O mode is usually the first oper-
ation after H_RESET or S_RESET. The RAP register
will point to CSR0 at that time. Writing a value of 0 to
CSR0 is a safe operation. DWIO (BCR18, bit 7) will be
set to 1 as an indication that the Am79C978 controller
operates in 32-bit I/O mode.
Table 25. I/O Map in Word I/O Mode (DWIO = 0)
No. of
Offset
00h - 0Fh
10h
Bytes
Register
16
2
APROM
RDP
RAP (shared by RDP and BDP)
Reset Register
BDP
Note: Even though the I/O resource mapping changes
when the I/O mode setting changes, the RDP location
offset is the same for both modes. Once the DWIO bit
has been set to 1, only H_RESET can clear it to 0. The
DWIO mode setting is unaffected by S_RESET or set-
ting of the STOP bit. Table 27 shows how the 32 bytes
of address space are used in DWord I/O mode.
12h
2
14h
2
16h
2
18h - 1Fh
8
Reserved
Double Word I/O Mode
All I/O resources must be accessed in DWord quanti-
ties and on DWord addresses. A read access other
than listed in Table 27 will yield undefined data, a write
operation may cause unexpected reprogramming of
the Am79C978 control registers.
The Am79C978 controller can be configured to operate
in DWord (32-bit) I/O mode. The software can invoke
the DWIO mode by performing a DWord write access
to the I/O location at offset 10h (RDP). The data of the
write access must be such that it does not affect the in-
tended operation of the Am79C978 controller. Setting
Table 26. Legal I/O Accesses in Word I/O Mode (DWIO = 0)
AD[4:0]
0XX00
0XX01
0XX10
0XX11
BE[3:0]
1110
Type
RD
Comment
Byte read of APROM location 0h, 4h, 8h, or Ch
Byte read of APROM location 1h, 5h, 9h, or Dh
Byte read of APROM location 2h, 6h, Ah, or Eh
Byte read of APROM location 3h, 7h, Bh, or Fh
1101
RD
1011
RD
0111
RD
Word read of APROM locations 1h (MSB) and 0h (LSB), 5h and 4h, 8h and 9h, or
Ch and Dh
0XX00
0XX10
1100
0011
RD
RD
Word read of APROM locations 3h (MSB) and 2h (LSB), 7h and 6h, Bh and Ah, or
Fh and Eh
10000
10010
10100
10110
1100
0011
1100
0011
RD
RD
RD
RD
Word read of RDP
Word read of RAP
Word read of Reset Register
Word read of BDP
Word write to APROM locations 1h (MSB) and 0h (LSB), 5h and 4h, 8h and 9h, or
Ch and Dh
0XX00
0XX10
1100
0011
WR
WR
Word write to APROM locations 3h (MSB) and 2h (LSB), 7h and 6h, Bh and Ah, or
Fh and Eh
10000
10010
10100
10110
1100
0011
1100
0011
WR
WR
WR
WR
Word write to RDP
Word write to RAP
Word write to Reset Register
Word write to BDP
DWord write to RDP,
switches device to DWord I/O mode
10000
0000
WR
Am79C978
97
AMD [ AMD ]
