Specifications

EPC-8 Hardware Reference
4 4
4-18
When using big-endian byte ordering, care must be taken to assure that the VME
address is aligned on a boundary; for D16 accesses the VME address must be on a
word boundary (address evenly divisible by 2) and for D32 accesses the VME address
must be on a double-word boundary (evenly divisible by 4). If this is not done, the
results will be "scrambled" data. Although the VMEbus address must be boundary-
aligned to match the data width (word or double-word), the 486 address does not need
to be boundary-aligned.
Another consideration is the compiler being used. Some compilers produce two
16-bit accesses when a 32-bit access is desired. When this occurs, again the data will
be "scrambled."
When transferring a 32-bit floating-point number, special care must be taken to assure
that both processors use the same floating-point format; that both systems expect the
mantissa and exponent in the same byte locations. As long as this is correct,
transferring a floating-point number will work correctly. Since transferring a 64-bit
floating-point number is not supported in hardware, two 32-bit transfers must be used
with little-endian byte order and then byte-swapping must be accomplished in
software.
The EPConnect Bus Manager software provides a means of selecting the byte
ordering during memory-copy operations.
VMEbus Interrupt Response
When the EPC-8's Interrupt Generator register (815F) is used to assert an interrupt, the
EPC-8 formulates a status/ID value that is transmitted on the bus as the response to a
matching interrupt acknowledge cycle. The EPC-8 acts as both a D08(O) and D16
interrupter. For D08 interrupt acknowledge cycles, the status/ID value is the EPC-8's
logical address (11111aaa, where aaa is the value of ULA as defined in port 814A).
For D16 and D32 interrupt acknowledge cycles, the status/ID value consists of 16 bits.
The upper eight bits are the upper half of the response register (the value in I/O port
814B) and the lower eight bits are the logical address.
VME Extension Registers (VXI)
The EPC-8 maps a standard set of VXI configuration registers onto the VMEbus A16
space and thus accessible by other VMEbus modules. These registers are 16-bit
registers occupying 64 bytes of A16 space at a base address defined by the EPC-8's
logical address. The VME-mapped registers are a subset of those defined as I/O ports
in the EPC-8. The registers are dual-ported in that they are accessible both from VME
and from within the EPC-8 as ports in its I/O space.
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