
SMSC DS – FDC37N958FR
Page 173
Rev. 09/01/99
Table 62 - Register Bit Allocation
D5
D4
D3
D7
D6
D2
D1
D0
x
x
x
x
x
x
x
GATEA20
Writing a "0" into location D0 causes the
GATEA20 output to go low, and vice versa.
When the register at location 7FFBH is read, all
unused bits (D7-D1) are read back as "0".
Host control and firmware control of GATEA20
affect two separate register elements. Read
back of GATEA20 through the use of 7FFBH
reflects the present state of the GATEA20 output
signal: if SAEN is set, the value read back
corresponds to the last firmware-initiated control
of GATEA20; if SAEN is reset, the value read
back corresponds to the last host-initiated
control of GATEA20.
Host control of the GATEA20 output is provided
by the hardware interpretation of the "GATEA20
sequence" (see Table 61). The foregoing
description
assumes
configuration bit is reset.
that
the
SAEN
When the FDC37N958FR receives a "D1"
command followed by data (via the host
interface), the on-chip hardware copies the value
of data bit 1 in the received data field to
the GATEA20 host latch. At no time during this
host-interface transaction will PCOBF or the IBF
flag (bit 1) in the Status register be activated;
i.e., this host control of GATEA20 is transparent
to firmware, with no consequent degradation of
overall system performance. Table 63 details
the possible GATEA20 sequences and the
FDC37N958FR responses.
On VCC1 POR, GATEA20 will be set.
An additional level of control flexibility is offered
via a memory-mapped synchronous set and
reset capability. Any data written to 7FFEH
causes the GATEA20 host latch to be set, while
any data written to 7FFFH causes it to be reset.
This control mechanism should be used with
caution. It was added to augment the "normal"
control flow as described above, not to replace
it. Since the host and the firmware have
asynchronous control capability of the host latch
via this mechanism, a potential conflict could
arise. Therefore, after using the 7FFEH and
7FFFH addresses, firmware should read back
the GATEA20 status via 7FFBH (with SAEN = 0)
to confirm the actual GATEA20 response.