11.3 ISA Bus Configuration Addresses (Read-Port etc.)
These addresses are also known as the "Auto-configuration Ports". For the ISA bus, there is technically no configuration address space, but there is a special way for the CPU to access PnP configuration registers on the PnP cards. For this purpose 3 @ I/O addresses are allocated and each addresses only a single byte (there is no "range"). This is not 3 addresses for each card but 3 addresses shared by all ISA-PnP cards.
These 3 addresses are named read-port, write-port, and address-port. Each port is just one byte in size. Each PnP card has many configuration registers so that just 3 addresses are not even sufficient for the configuration registers on a single card. To solve this problem, each card is assigned a card number (handle) using a technique called "isolation". See ISA Isolation for the complex details.
Then to configure a certain card, its card number (handle) is sent out via the write-port address to tell that card that it is to listen at its address port. All other cards note that this isn't their card number and thus don't listen. Then the address of a configuration register (for that card) is sent to the address-port (for all cards --but only one is listening). Next, data transfer takes place with that configuration register on that card by either doing a read on the read-port or a write on the write-port.
The write-port is always at A79 and the address-port is always at 279 (hex). The read-port is not fixed but is set by the configuration software at some address (in the range 203-3FF) that will hopefully not conflict with any other ISA card. If there is a conflict, it will change the address. All PnP cards get "programmed" with this address. Thus if you use say isapnp to set or check configuration data it must determine this read-port address.
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