Memory Addresses, Device IRQ

Device IRQ

0 System timer

1 Keyboard controller

2 Reserved (not used)

3 COM2

4 COM1

5 LPT2

6 Floppy drive controller

7 LPT1

When the 16-bit ISA bus appeared, more IRQs became available. A second interrupt controller chip was added to the systemboard chip set, which hooked to the first controller. The second controller used one of the first controller’s IRQ values (IRQ 2) to signal the first controller (see the figure below). But there was a problem with that because there were some devices that used IRQ 2. Tying the new IRQ 9 to the old IRQ 2 pin on the 16-bit ISA bus solved the problem. The result was that a device could still use the pin on the expansion slot for IRQ 2, but it is really IRQ 9. (Look carefully at the figure below to confirm that.) Because of this, the priority level becomes: 0, 1, (8, 9, 10, 11, 12, 13, 14, 15), 3, 4, 5, 6, 7.

To see how the IRQs are assigned on your computer, use MSD for DOS and Windows 3.x, and Device Manager for Windows 9x. For Windows 9x, click Start, Programs, Settings, and Control Panel, and double-click System. Click the Device Manager tab. Select Computer and click Properties. The figure below shows the Computer Properties dialog box. Notice that IRQ 2 is assigned to the programmable interrupt controller, and IRQ 9 is used by the video card.

Memory Addresses

Once the IRQ gets the attention of the CPU, its job is done, but memory addresses are used as the device is serviced. Recall that memory addresses are numbers assigned to both ROM and RAM memory so that the CPU can access that memory. Think of memory addresses as a single long list of hexadecimal numbers. The CPU has a fixed number of memory addresses available to it as determined by the CPU and the bus it is using. These memory addresses can be assigned to any type of physical memory in the system that needs to be addressed by the CPU, including ROM and RAM chips on expansion cards and ROM and RAM chips on the systemboard, which can hold either data or instructions. Once addresses have been assigned, the CPU only sees this physical memory as a single list that can be accessed by using the memory addresses. Before the CPU can process data or instructions, both must be in physical memory, and the physical memory must be assigned memory addresses. In the case of ROM BIOS, these programs are already located on a memory chip, and so the only thing needed is for memory addresses to be assigned to them. Software stored on a hard drive or other secondary storage device must first be copied into RAM before processing, and memory addresses must be assigned to that RAM.

Data coming from a keyboard or some other input device or stored in data files on a hard drive must also be copied into RAM before processing, and memory addresses must be assigned to that RAM. As one example of this process, the figure below shows that both RAM and memory addresses are needed so the CPU can load a program stored on the hard drive into memory before executing it.

Ways Memory Addresses Are Used

The table below shows the ways that the CPU uses its list of memory addresses. Startup BIOS and System BIOS—also called on-board BIOS—stored in the ROM-BIOS chip on the systemboard must be assigned memory addresses so that the CPU can access these programs. RAM stored in SIMMs and DIMMs on the systemboard makes up the bulk of memory that is used by the CPU and uses the lion’s share of available memory addresses. Many programs and data are copied into this RAM, including device drivers, portions of the OS, and applications software and data.