A Look Back: ISA

The ISA bus is quite old in design and is a notoriously poor performer, but it still holds a good part of the market for extension devices. If speed is not important and you want to support old motherboards, an ISA implementation is preferable to PCI. An additional advantage of this old standard is that if you are an electronic hobbyist, you can easily build your own ISA devices, something definitely not possible with PCI.

On the other hand, a great disadvantage of ISA is that it’s tightly bound to the PC architecture; the interface bus has all the limitations of the 80286 processor and causes endless pain to system programmers. The other great problem with the ISA design (inherited from the original IBM PC) is the lack of geographical addressing, which has led to many problems and lengthy unplug-rejumper-plug-test cycles to add new devices. It’s interesting to note that even the oldest Apple II computers were already exploiting geographical addressing, and they featured jumperless expansion boards.

Despite its great disadvantages, ISA is still used in several unexpected places. For example, the VR41xx series of MIPS processors used in several palmtops features an ISA-compatible expansion bus, strange as it seems. The reason behind these unexpected uses of ISA is the extreme low cost of some legacy hardware, like 8390-based Ethernet cards, so a CPU with ISA electrical signaling can easily exploit the awful but cheap PC devices.

Hardware Resources

An ISA device can be equipped with I/O ports, memory areas, and interrupt lines.

Even though the x86 processors support 64 kilobytes of I/O port memory (i.e., the processor asserts 16 address lines), some old PC hardware decodes only the lowest 10 address lines. This limits the usable address space to 1024 ports, because any address in the range 1 KB to 64 KB will be mistaken for a low address by any device that decodes only the low address lines. Some peripherals circumvent this limitation by mapping only one port into the low kilobyte and using the high address lines to select between different device registers. For example, a device mapped at 0x340 can safely use port 0x740, 0xB40, and so on.

If the availability of I/O ports is limited, memory access is still worse. An ISA device can use only the memory range between 640 KB and 1 MB and between 15 MB and 16 MB. The 640-KB to 1-MB range is used by the PC BIOS, by VGA-compatible video boards, and by various other devices, leaving little space available for new devices. Memory at 15 MB, on the other hand, is not directly supported by Linux, and hacking the kernel to support it is a waste of programming time nowadays.

The third resource available to ISA device boards is interrupt lines. A limited number of interrupt lines are routed to the ISA bus, and they are shared by all the interface boards. As a result, if devices aren’t properly configured, they can find themselves using the same interrupt lines.

Although the original ISA specification doesn’t allow interrupt sharing across devices, most device boards allow it.[60] Interrupt sharing at the software level is described in Section 9.6,” in Chapter 9.

ISA Programming

As far as programming is concerned, there’s no specific aid in the kernel or the BIOS to ease access to ISA devices (like there is, for example, for PCI). The only facilities you can use are the registries of I/O ports and IRQ lines, described in Section 2.5 (Chapter 2) and Section 9.3 (Chapter 9).

The programming techniques shown throughout the first part of this book apply to ISA devices; the driver can probe for I/O ports, and the interrupt line must be autodetected with one of the techniques shown in Section 9.3.2,” in Chapter 9.

The helper functions isa_readb and friends have been briefly introduced in Section 8.4 in Chapter 8 and there’s nothing more to say about them.

The Plug-and-Play Specification

Some new ISA device boards follow peculiar design rules and require a special initialization sequence intended to simplify installation and configuration of add-on interface boards. The specification for the design of these boards is called Plug and Play (PnP) and consists of a cumbersome rule set for building and configuring jumperless ISA devices. PnP devices implement relocatable I/O regions; the PC’s BIOS is responsible for the relocation—reminiscent of PCI.

In short, the goal of PnP is to obtain the same flexibility found in PCI devices without changing the underlying electrical interface (the ISA bus). To this end, the specs define a set of device-independent configuration registers and a way to geographically address the interface boards, even though the physical bus doesn’t carry per-board (geographical) wiring—every ISA signal line connects to every available slot.

Geographical addressing works by assigning a small integer, called the Card Select Number (CSN), to each PnP peripheral in the computer. Each PnP device features a unique serial identifier, 64 bits wide, that is hardwired into the peripheral board. CSN assignment uses the unique serial number to identify the PnP devices. But the CSNs can be assigned safely only at boot time, which requires the BIOS to be PnP aware. For this reason, old computers require the user to obtain and insert a specific configuration diskette even if the device is PnP capable.

Interface boards following the PnP specs are complicated at the hardware level. They are much more elaborate than PCI boards and require complex software. It’s not unusual to have difficulty installing these devices, and even if the installation goes well, you still face the performance constraints and the limited I/O space of the ISA bus. It’s much better in our opinion to install PCI devices whenever possible and enjoy the new technology instead.

If you are interested in the PnP configuration software, you can browse drivers/net/3c509.c, whose probing function deals with PnP devices. Linux 2.1.33 added some initial support for PnP as well, in the directory drivers/pnp.

[60] The problem with interrupt sharing is a matter of electrical engineering: if a device drives the signal line inactive—by applying a low-impedance voltage level—the interrupt can’t be shared. If, on the other hand, the device uses a pull-up resistor to the inactive logic level, then sharing is possible. This is nowadays the norm. However, there’s still a potential risk of losing interrupt events since ISA interrupts are edge triggered instead of level triggered. Edge-triggered interrupts are easier to implement in hardware but don’t lend themselves to safe sharing.

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