The 8-bit and 16-bit ISA Bus, Interrupt Request Number

The 8-bit and 16-bit ISA Bus

All devices are directly or indirectly connected to the systemboard because they are all dependent on the CPU for processing their data. A device connects to the systemboard by a data cable, a slot, or a port coming directly off the systemboard. In any case, the device always connects to a single bus on the systemboard. Recall that there are several different buses on a systemboard, but our discussions here will be limited to only one bus, an older bus used on the early PCs of the 1980s, called the ISA (Industry Standard Architecture) bus, because this simpler bus is easier to understand. The first ISA bus had only eight lines for data and was called the 8-bit ISA bus.

Some of the lines on the bus are used for data, addresses, and voltage, and others are a variety of control lines. Eight lines are used for data, and twenty lines are used for addresses. These 20 lines can carry either memory addresses or I/O addresses. The CPU determines which type of address is using these lines by setting control lines B11 through B14 (memory read/write and I/O read/write). Since this bus has only 20 address lines, the largest address value that can travel on the bus is 1111 1111 1111 1111 1111 or 1,048,576 (1024K).

Two lines are required to manage a DMA channel: DRQ (Direct Request) and DACK (Direct Acknowledge). There are four DMA channels (0, 1, 2, and 3) on the 8-bit ISA bus.

As computer technology improved, the demand increased for more memory, more devices to be operating at the same time, and faster data transfer, making it necessary to provide more memory addresses, DMA channels, and IRQs. The 16-bit ISA bus was invented to meet these requests. The 16-bit ISA bus added an extra extension to the 8-bit slot allowing for eight additional data lines (total of 16), five additional IRQ lines, four more DMA channels, and four additional address lines (total of 24). Today, the 16-bit ISA bus is still used on systemboards, although newer, faster buses are also used. An 8-bit expansion card (an expansion card that only processes 8 bits of data at one time) can use the 16-bit ISA expansion slot. It only uses the first part of the slot.

Systemboards also contain other newer buses that are faster and provide more options, but the basics haven’t changed. You can still find lines on these buses for data, addresses, IRQs, and DMA channels, although it is common practice today for a line to perform several functions, making it not quite as easy to study the bus.

Interrupt Request Number

When a hardware device needs the CPU to do something, such as when the keyboard needs the CPU to process a keystroke after a key has been pressed, the device needs a way to get the CPU’s attention and the CPU must know what to do once its attention is turned to the device. These interruptions to the CPU are called hardware interrupts and the device handles them by placing voltage on a designated line on the bus it is connected to. These lines are numbered, and a line is referred to as an interrupt request number, or IRQ. This voltage on the line serves as a signal to the CPU that the device has a request that needs processing. Often, a hardware device that needs attention from the CPU is referred to as “needing servicing.” Look carefully at the figure below which shows IRQ lines (IRQ 2, 3, 4, 5, 6, and 7). There are actually eight IRQs built into this bus, but IRQs 0 and 1 are not available for expansion cards (because they are used for system timer and keyboard), so they are not given a pin on the expansion slot. IRQ 2 was reserved in the early days of PCs because it was intended to be used as part of a link to mainframe computers. Thus, only five IRQs were available for devices, and each device had to have its own IRQ. This made it difficult for more than five devices to be connected to a PC at any one time. COM1 and COM2 are preconfigured assignments that can be made to serial devices such as modems, and LPT1 and LPT2 are preconfigured assignments that can be made to parallel devices such as printers.

On early systemboards, the eight IRQs were managed by an Intel microchip called the interrupt controller chip and labeled the Intel 8259 chip. This chip had a direct connection to the CPU and signaled the CPU when an IRQ was activated. The CPU actually doesn’t know which IRQ is “up” because the interrupt controller manages that for the CPU. If more than one IRQ is up at the same time, the interrupt controller selects the IRQ that has the lowest value to process first. For example, if a user presses a key on the keyboard at the exact same time that she moves the mouse installed on COM1, since the keyboard is using IRQ 1 and the mouse on COM1 is using IRQ 4, the keystroke is processed before the mouse action. The table below shows some of the common IRQs for devices using the early 8-bit ISA bus.