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):

• Memory addresses
• IRQ requests
• DMA channels
• I/O port addresses

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

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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.

ISA Plug-and-Play (PnP 1.0A 7)

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 that 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 a 32-byte sequence sent to the ADDRESS port that will put the cards in configuration mode. Send zero twice to clear any previous address before sending this key.

; Warning: not tested
SendKey:
 pushfd            ; Save IF
 cli
 mov    dx,ADDRESS
 mov    esi,Key
 mov    ecx,32+2
 rep    outsb
 popfd             ;  Restore IF, nothing else changed
 ret

The key is defined as:

Key:
 ; Zero bytes to clear ADDRESS
 DW     0
 ; The actual key
 DB     0x6A,0xB5,0xDA,0xED,0xF6,0xFB,0x7D,0xBE
 DB     0xDF,0x6F,0x37,0x1B,0x0D,0x86,0xC3,0x61
 DB     0xB0,0x58,0x2C,0x16,0x8B,0x45,0xA2,0xD1
 DB     0xE8,0x74,0x3A,0x9D,0xCE,0xE7,0x73,0x39

AMD recommends (PCnet-ISA II 3) disabling interrupts while performing the IO operation to prevent anything else from interfering (as shown above). 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 should be obtained using the PnP BIOS. CSNs are always sequential and start from zero.

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.