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. The goal of this section is to clarify the documentation as there are many forward references to the appendices. 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
 cld
 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 one (zero represents unidentified cards).

Configuration Space

Appendix A specifies the actual configuration space. Here is an overview of what the card control section looks like:

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

Offset	Name	What it does
0x00	Set READ_DATA	Set the READ_DATA address
0x01	Serial Isolation	(TODO: Add this)
0x02	Config Control	See below for spec
0x03	Wake[CSN]	Writing here wakes that CSN

Note: Some operations appear to be global is scope

If it is not clear, writing to the Wake[CSN] port sets the actual CSN to perform the wake operation.

The Set READ_DATA port (also referred to as RD_DATA) is an 8-bit value that relocates the READ_DATA port to the range specified above. It is practically shifted left by three and or'ed with the 10-bit binary number "11" (Page 15). The value written to this register would be:

Set_READ_DATA = (PhysicalPort & 0x3F8) >> 3.

Reading this register does nothing, the programmer must assign this port and check for conflicts. There is only one for all cards.

Isolation Protocol (PnP 3.3.2)

Directly after the initiation key has been sent and all cards have been switched from wait for key to sleep mode, the isolation protocol must be performed. It is at this point where the READ_DATA port should be assigned.

When the initiation key is sent, all cards are placed in sleep mode. The Wake[CSN] command is used to place cards in config mode. If the parameter is zero, all cards are placed in isolation mode instead. The system software must try to set the READ_DATA port address and read 72 bytes from it, and check each pair of reads for 0x55 and 0xAA. If this sequence is not detected and there is a checksum mismatch (TODO: more info), the port is in conflict and must be reassigned. If the OS cannot find a place to place the READ_DATA port, PnP software is to assume there are no plug-and-play ISA cards installed on the system, so no further work is needed.

Resource Configuration

Config Control Register

Index	Purpose
0	Reset all logical devices
2	Reset CSN to zero

Toggling bits in the CCR will perform several commands. The bits are reset once the task is performed.

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.