ISA
- Industry Standard Architecture, ISA may also refer to Instruction Set Architecture.
The Industry Standard Architecture (ISA) bus was created for the original IBM PC back in 1981. At that stage, it was an 8-bit, 5MHz bus (2.39MB/s), but was later upgraded to 16-bits at 8MHz (8.33MB/s). Today, the ISA bus is archaic and incredibly slow, but it is still commonly found in older machines, and many of the most common, basic devices are connected to it. For this reason, it is still supported by many operating systems. It is slowly being replaced by the Super I/O chip that is common amongst modern machines. For more information on the history and implementation details of the bus, visit the Wikipedia page on the subject.
Device Conflicts
There are broadly four types of resources that cause device conflicts (listed in the order problems are generally experienced):
Managing these resources becomes the responsibility of the operating system. Fortunately, Microsoft has created a standard called Plug-and-Play (PnP) that helps with this. Not all ISA cards support it, however, so some luck is required in that the person building the system has set the correct jumpers.
ISA Plug-and-Play Overview
The following is a general overview of the ISA Plug-and-play specification. See the original document (ISA PnP 1.0A) for more authoritative information (including timeouts). There are three 8-bit ports specified by the ISA PnP standard that are used for accessing configuration spaces.
| Address | Name | R/W |
|---|---|---|
| 0x279 (Same is LPT status, but W) | ADDRESS | W |
| 0xA79 | WRITE_DATA | W |
| 0x203-0x3FF (Relocatable) | READ_DATA | R |
Note: ISA ports use 12-bit decode (PnP 1.0A 7)
The ADDRESS port is an 8-bit port but takes two bytes. The first byte send is the CSN, or card select number. The second is the offset in the configuration space. The address is the same as the printer status, but ports are bidirectional on PCs.
Cards in PnP mode are disabled on startup and are in a "Wait for Key" state. Once configured, they should be returned to this state.
The Initiation Key is the following 32-byte sequence. It is sent to the ADDRESS port. Send zero twice to clear any previous address before sending this key.
6A,B5,DA,ED,F6,FB,7D,BE, DF,6F,37,1B,0D,86,C3,61, B0,58,2C,16,8B,45,A2,D1, E8,74,3A,9D,CE,E7,73,39
AMD recommends (PCnet-ISA II 3) disabling interrupts while performing the IO operation to prevent anything else from interfering. No other ports should be accessed. It is also recommended to send it twice in case the key is not received the first time.
Before doing anything with ISA cards, the number of CSNs must be obtained using the PnP BIOS. CSNs are always sequential.
External Links
PCnet-ISA II programming guide. This has some useful information on PnP. https://www.amd.com/system/files/TechDocs/PCnet-ISA_II.pdf#:~:text=The%20Initiation%20Key%20places%20the%20PCnet-ISA%20II%20Plug,the%20Plug%20and%20Play%20auto-configuration%20ports%20are%20enabled.