Vax 613 0S Instruction Manual download pdf (Page 148)

9.1 I/O Read and Write Restrictions

Several restrictions apply to I/O operations.

• Read operations performed on reserved locations or read operations performed

on write-only bits, result in UNPREDICTABLE data being returned.

• Write operations performed on reserved locations or write operations

performed on read-only bits result in UNPREDICTABLE data being written

to UNPREDICTABLE locations.

• The use of both dense and sparse I/O space means that performing a read

operation from a longword register through Dense I/O Space results in two

longword I/O read operations on the TURBOchannel, because you are reading

two I/O locations instead of one.

• You must not access the CXTurbo in Sparse I/O Space and so cannot perform

byte-masked operations to the CXTurbo, if you are programming I/O for the

300 models.

• The 300 models TURBOchannel runs at 12.5 MHz. Initialize the register of

a device that requires this information (for example, SSR<28> of the IOCTL

ASIC), before using the device.

• The 300 models TURBOchannel does not support parity. The drivers of

devices connected to the TURBOchannel must initialize the devices to a

non-parity mode of operation.

• The 300 models do not support block-mode I/O write operations.

9–2 I/O Programming

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Vax 613 0S Instruction Manual Page 148