Instruction manual

6.6 System FEPROM (500 Models)
Half of system ROM in the 500/500S models is addressable through the SFB ASIC
at Dense I/O space locations 1.E200.0000-1E20F.FFFF. It is also addressable
through Sparse I/O space.
For both read and write operations, each ROM byte is mapped to a longword in
I/O space. For example, to read ROM byte number 1, you must perform a read
operation to Dense I/O space offset 1.E200.0004.
Longword read operations to these addresses return a single byte in the low order
byte. The remaining 3 bytes are UNPREDICTABLE.
For write operations, the low order byte is always the byte that is written to the
ROM. For system ROM write operations, the TURBOchannel bytemask portion of
the address is ignored. Only 1 byte is written to the ROM for one TURBOchannel
write operation.
Write operations to the CXTurbo portion of the system ROM may be disabled by
removing the CXTurbo security jumper.
There is no parity in the system ROM. Since four bytes cannot be packed into
TURBOchannel longwords, ROM code is non-executable, unless a separate
routine packs these bytes into memory before execution. In addition, the code in
ROM may be in compressed form and require uncompression.
6–24 CXTurbo Graphics Subsystem: 300/500 Models