Status And Control Registers - HP 9835A Programming Manual

136 1/0 Handling
Pa holds a binary number in the range 0 to 15 (utilizing only the lower four bits of the word, 0 to
3). Each interface has an externally-settable select code switch which can also be set to a value
between 0 and 15. However, since select codes 0 and 15 are reserved for the internal printer
and tape cartridge unit, respectively, the permissible select code settings are 1 through 14.
Whenever an operation to one of the 1/0 registers is performed, the 9835A/B makes the
contents of the Pa register available to all the interfaces connected to the backplane. Each card
compares the value with its own select code. If they match, the interface responds to the
operation.
So, for example, if the following statements are executed in turn -
I ::::;CUF'CE LDj::"l ... ,.,
I
:::;Ci!...lF"~C:E ,:::;oon=i
eo;:;'
I ':;()i..JF:CE L..DFi ~>!.
then a word of data is read from the interface card set to select code 8. (The data was read in the
third line; this is discussed in "Programmed 1/0" below.)
The label "Pa" is reserved by the assembler for the Peripheral Address register.
Status and Control Registers
The primary purpose of any interface is to allow data to be exchanged between the computer
and the peripheral device to which it is connected. But HP's 98030 series of interface cards are
even more versatile, possessing a programmable capability of their own. This in turn provides
optional capabilities with the card that can be set and changed by control instructions from the
9835A/B. (For details on what capabilities are provided, consult the Interfacing Concepts
manual.)
The programming of the interface is done by the 9835A/B using the R5 register. Some of the
interfaces use other registers for extended control bits (these are also described in the Interfac-
ing Concepts manual).
Interface cards can also return information to the 9835AI B about which optional programming
features are currently selected. This information, called the status byte, is obtained through an
input operation using register R5. The status byte (8 bits) is determined solely by the charac-
teristics of the interface card being addressed in the Pa register. Again, information on particu-
lar cards can be found in Interfacing Concepts}.