HP E1433A User's Guide
Register Definitions
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:
RAM 0-1
Send Data
Receive Data
Query Response Command
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:
1Always read all 32 bits of a 32-bit register.
2Always 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:
1Always write all 32 bits of a 32-bit register.
2Always write the least significant part last.
