Module Overview
This module’s purpose is to interface the GSC (System Connect) bus to the EISA bus, giving the system a
standard expansion I/O bus. But because of EISA’s complexity, this module does not directly generate
EISA bus signals; it generates an i486-like bus, which the “EBCU” (EISA Bus Control Unit) and
“EPCU” (EISA Peripheral Control Unit) of TI’s EISA chip set then convert into EISA (and ISA). Thus, in
its primary mode of operation, this is an i486 bus conversion module rather than an EISA conversion
module.
Basic Operation
This conversion module supports the following functions:
CPU reads from and writes to (E)ISA I/O addresses—also note that address scrambling is
performed for accesses to ISA I/O space, so that each ISA board gets its own protectable page in
the CPU’s address space.
CPU reads from and writes to (E)ISA memory addresses—only 55.5 MB out of (E)ISA’s 4 GB
address space is accessible to the CPU, but the accessible ranges include subsets of each of ISA
20-address-bit, ISA 24-address-bit, and EISA 32-address-bit memory.
CPU reads from and writes to the address map RAM—after the CPU sets it up, this mapper lets
(E)ISA master and DMA devices access any desired pages in system RAM.
CPU reads from and writes to the Lock Control Register—this register lets the CPU run a
sequence of “locked” (undivided) cycles on the EISA bus.
CPU reads from and writes to the FIFO Enable Register—this register lets the CPU flush the
inbound buffer and clear the outbound buffer, or disable data buffering altogether.
CPU reads from the Interrupt Acknowledge Register—this is the way the CPU runs interrupt
acknowledge cycles to the (E)ISA interrupt controller and obtains the “interrupt vector” number
of the highest pending (E)ISA interrupt.
(E)ISA master and DMA reads from and writes to the mapped address space—these accesses are
passed through to the system RAM, after having their addresses translated by the appropriate
address map entries. If these accesses are not “locked” on the EISA bus, they can be buffered:
writes do not immediately occur to system RAM, and reads can be satisfied from data which was
pre-fetched from system RAM. If these accesses are “locked” by the assertion of EISA’s
LOCK
signal, the lock is propagated all the way back to system RAM, and no other masters can run bus
cycles or access system RAM between the locked EISA cycles.
Interrupt requests and non-maskable interrupts from (E)ISA—after flushing and clearing the data
FIFO to ensure that the system sees consistent data, these are simply passed to the interrupt
controller where they are dealt with appropriately.
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