ISA: Difference between revisions
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* 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.
== Decode Problems ==
The ISA bus can support x86 16-bit port addresses, but many older ISA cards only support 10-bit address decode. This presents a major problem that can lead to conflict. When the processor tries to access a certain resource on the ISA bus, it will signal all ISA devices in the hopes that only one responds. If a card can only handle ten of the address bits, it can respond to IOR and IOW signals when it should not be, because it does not see the upper bits.
So if a device is located at port 100H, that 10-bit address with every possible combination for the upper 6 bits will map to the same device (e.g port 0xFF00). To make this totally impossible, one can simply restrict PnP port addresses to be within the 1024-byte range to make such conflicts impossible, though this would be a limitation. It is also important to note that PCI uses 0CF8-0xCFF, which is above the 10-bit range and cannot be reassigned. Built-in devices reported by the PnP BIOS such as the PCI host bridge should the address range with the top 6 bits masked out. This way, ISA cards cannot be configured in a conflicting manner.
Conflicts are not restricted to IO and should apply to memory too, but any mainboard with an ISA bus and more than 16MB of RAM will probably be smart enough to check addresses before sending them to the ISA bus. They may check IO port addresses too, but it is always best to verify rather than trust 90's era hardware that sometimes had dubious standard compliance.
The decode of legacy devices like the LPT, COM, etc. that are typically embedded in the mainboard are probably 16-bit on PnP boards, but checking with the BIOS is the only way to be sure.
= ISA Plug-and-Play Overview =
The original ISA bus, or more specifically, the 16-bit PC/AT bus used static non-sharable resource assignments and had no isolation between cards. There was no way to program it from software. A number of
== I/O Ports and Initiation ==
▲The original ISA bus, or more specifically, the 16-bit PC/AT bus used static non-sharable resource assignments and had no isolation between cards. There was no way to program it from software. A number of busses appeared on the PC market such as VLB, PCI, and EISA. These architectures were incompatible with the original design. Plug-and-play ISA was introduced to get around the original limitations while maintaining compatibility and improving user friendliness.
There are three 8-bit ports specified by the ISA PnP standard that are used for accessing configuration spaces.
▲Programmers looking for robust support for the 1990's generation of hardware may want to add plug-and-play ISA support.
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'''Note:
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.
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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. Although not mentioned in the original specification, it is also recommended by AMD to '''send it twice''' in case the key is not received the first time.
The specification details isolation of cards. BIOS is supposed to perform this procedure. The term "plug-and-play software" in the specification may refer to the OS and firmware. On startup, the BIOS will send the initiation key, perform the serial isolation procedure, and take the cards out of isolation or configuration mode and have them ''wait for key''. Each card is assigned a sequential handle starting from 1 called a CSN. The number of CSNs that the BIOS assigned, as well as the READ_DATA port that the BIOS decided can be found with PnP function 40h.
Despite the BIOS performing isolation itself, the PnP BIOS specification says that the OS should not rely on the BIOS at all and do it again.
== Card Control Space ==
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| 0x02 || Config Control || See below for spec
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| 0x03 || Wake[CSN] |
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| 0x04 || Resource Data|| Reading fetches the next byte
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| 0x06 || Card Select Number || The CSN, can be changed
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| 0x07 || Logical Device || Selects a logical device, read-only zero if none
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| 0x08-1F||Card reserved || ~
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=== Configuring READ_DATA ===
The Set READ_DATA
Set_READ_DATA = (PhysicalPort & 0x3F8) >> 3 /* Make sure first bits are 11 */
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=== Resource Data ===
Resource data is read from the dedicated register for the logical device selected. Bit zero of the status register must be polled until it is one before reading the resource data. The resource data register is READ ONLY. This resource data is not to be confused with the resource configuration registers in the card control space. It defines what resources the card '''is able''' to use. If a fully PnP card uses any memory address range, the static resource entry will be 0x00000000-0xFFFFFFFF (AFAIK), and the same applies to ports, DMA, and IRQs. If only one DMA or IRQ is implemented, that device must use it. For IO ports, the tag can be a fixed descriptor or regular one. If the tag is a fixed descriptor, the old 10-bit ISA decode is used for that range.
The PnP ISA specification allows for 32-bit data and addresses. This is definitely not used by any ISA cards because the ISA bus is 16-bit and only supports 24-bit addresses. Despite this, the OS should support this by indirection for consistency.
The PnP BIOS uses the same format for reporting devices connected to the mainboard.
== Logical Device Local Control ==
The logical card control registers are found at 0x30-0x3F. This space is present on all cards, even if they do not have any logical devices. Selecting the logical device swaps out this space. To check if the card is multi-function, write a value other than zero to the logical device register and try to write it back. If a read of LDS still generates a zero, the device has no functions. Otherwise, write back the original value (zero i.g.).
=== Activation ===
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The activate register turns on the card or the logical device belonging to that card when the first bit is on. If there are no resources for a card, the OS will have to disable the card by setting the register to zero (the user should be notified). IO range check should be disabled while activating.
== Resource Configuration ==▼
▲== Isolation Protocol (PnP 3.3.2) ==
The resource data is stored in the logical card space. The memory information starts at 0x40 or 0x76 and is seen at Table A-3. Memory descriptors are four bytes for 24-bit addressing and nine for 32-bit. The specifics are detailed in A.3.1.
== Supporting ISA PnP ==
PnP ISA devices can suport varrying levels of configurability. For example, a card that implements a parallel port may only be configurable to three standard addresses for PC compatibility. The static resource data mentioned previously reports what data a device supports, but not necessarily what it will use.
▲== Resource Configuration ==
The plug-and-play management code of the OS must be able to handle every possible bus, and combinations of buses, as some computers have PCI and ISA. Reserving resources should be supported so that less-so or non-configurable devices can work.
The following specifics of ISA PnP must be implemented in the operating system:
* Memory address ranges supported
* IO port ranges
* IRQs
* Address and port decode
* Data width (8/16/32-bit)
[[Category:Buses]]
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=External Links=
*[http://www.osdever.net/documents/PNP-ISA-v1.0a.pdf Tutorial on how to enable PCnet-ISA II PnP]
*[https://www.amd.com/system/files/TechDocs/PCnet-ISA_II.pdf
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