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q Parameter 1-7 Registers: These are 32-bit RAM locations used to pass parameters
along with commands to the device or query responses from the device. See the
following section regarding D16/D08 access of 32-bit registers and the
communication protocol.
32-bit Registers
Several of the A16 registers (and all other 24-bit registers) are implemented
as 32-bit-only resources. These are accessible using VME Bus D16 and
D08(EO) accesses. However certain restrictions apply. The affected A16
registers are:
q RAM 0-1
q Send Data
q Receive Data
q Query Response Command
q Parameter 1-7
Reading 32-bit Registers
When reading a 32-bit register using 8- or 16-bit modes, a simple caching
mechanism is used. On any read including the most significant byte (lowest
address), the 32-bit register is read and all 32-bits are latched into the read
cache. A read not including the most significant byte fetches data from the
read cache, without re-reading the register. This insures that the data will
be unchanged by any intervening write by the DSP (which would result in
garbled data).
This mechanism also introduces a hazard. Reads of less significant bytes
get data from the 32-bit register last read by a most-significant-byte read.
In other words, you can’t read the least significant byte first, or by itself.
Thus there are two important rules:
1 Always read all 32 bits of a 32-bit register.
2 Always read the most significant part first.
Writing 32-bit Registers
When writing to a 32-bit register using 8- or 16-bit modes, a simple caching
scheme is also employed. On any write not including the least significant
byte (highest address), the data is latched into the write cache. A write to
the least significant byte causes the cached data to be written to the 32-bit
register (in parallel with the current data for the least significant bytes(s).
This mechanism has its own hazards. Writes to the least significant byte
will always include the most recently cashed data, whether intended for
that register or not. Lone writes to the most significant part of a 32-bit
register will be lost if not followed by a write to the least significant part of
the same register. Thus there are two important rules:
1 Always write all 32 bits of a 32-bit register.
2 Always write the least significant part last.
HP E1433A User's Guide
Register Definitions
A-10