EISA System Architecture

A15

A14

A13

A12

A9

A8

AEN(from DMAC)

M/IO#

EISA

System

Board

I/O

Decoder

AEN0

AEN1

AEN2

AEN3

AEN4

AEN5

AEN6

AEN7

AEN8

AEN9 AEN10 AEN11

AEN12

AEN13

AEN14

AEN15

Figure 9-2. The System Board's AEN Decoder

EISA Product Identifier

EISA expansion boards, embedded devices and system boards have a four byte product ID that can be read from I/O port addresses xC80h – xC83h, where x = 0 for the system board or the number of the expansion slot the card is installed in. For example, the system board's ID can be read from I/O addresses 0C80 – 0C83h and slot one’s ID can be read from 1C80 – 1C83h.

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Chapter 9: EISA System Configuration

The first two bytes of the system board ID, read from I/O ports xC80 – xC81, contain a three character manufacturer's code. The three character manufacturer code is uppercase, ASCII alpha chosen by the manufacturer and registered with the firm that distributes the EISA spec. A compressed version of the ASCII code, using just the lower five bits of each character, is used. The third byte and the high-order four bits of the fourth byte are used to specify a product identifier consisting of three hex digits. The lower four bits of the fourth byte is use to specify the product revision number. Table 9-6 illustrates the format of the product ID bytes read from an expansion board. Table 9-7 illustrates the format of the product ID bytes read from an EISA system board.

To verify that an EISA expansion card is installed in a particular card slot:

•Write FFh to I/O port xC80h.

•Read one byte from xC80h.

•If the byte read equals FFh, an EISA card isn't installed in the slot. If the byte does not equal FFh and bit 7 of the byte read is zero, the card's EISA product ID can be read from xC80h – xC83h.

				Table 9-6. Expansion Board Product ID Format
	Location/Bits			Specify
	xC80, bit 7			not used, must be 0
	xC80, bits 6:2			1st compressed ASCII character of Manufacturer's ID
	xC80, bits 1:0			upper two bits of 2nd compressed ASCII character of Manufac-
				turer's ID
	xC81, bits 7:5			lower three bits of 2nd compressed ASCII character of Manufac-
				turer's ID
	xC81, bits 4:0			3rd compressed ASCII character of Manufacturer's ID
	xC82, bits 7:4			upper hex digit of product type
	xC82, bits 3:0			middle hex digit of product type
	xC83, bits 7:4			lower hex digit of product type
	xC83, bits 3:0			single hex digit of product revision number

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EISA System Architecture

				Table 9-7. EISA System Board Product ID Format
	Location/Bits			Specify
	0C80, bit 7			not used, must be 0
	0C80, bits 6:2			1st compressed ASCII character of Manufacturer's ID
	0C80, bits 1:0			upper two bits of 2nd compressed ASCII character of Manufac-
				turer's ID
	0C81, bits 7:5			lower three bits of 2nd compressed ASCII character of Manufac-
				turer's ID
	0C81, bits 4:0			3rd compressed ASCII character of Manufacturer's ID
	0C82, bits 7:0			reserved for manufacturer's use
	0C83, bits 7:3			reserved for manufacturer's use
	0C83, bits 2:0			EISA bus version

EISA Configuration Registers

In an ISA machine, expansion cards are configured by setting DIP switches and/or jumpers to the desired settings. This allows the user to select options such as:

•the start address of a device ROM mounted on the card

•the start address of RAM located on the card

•the IRQ line the card utilizes

•the DMA channel the card utilizes

•the I/O address range the card responds to

Setting the switches and/or jumpers allows the user to resolve conflicts between installed expansion cards. In addition, many ISA system boards have switches and/or jumpers that are used to configure the system board options.

The EISA specification replaces the switches and/or jumpers with special I/O locations. Each of these I/O locations can contain up to eight bits that may be used to select options on the system or expansion card. Each I/O location may be thought of as a pseudo-DIP switch bank. They are configuration registers. These special I/O locations reside in the slot-specific I/O address space starting at xC80h and extending up to xCFFh, a total of 128 locations. The first four of these I/O locations are reserved for the card ID, while three of the eight bits in xC84h are reserved for special card functions. The remaining five bits in xC84h and locations xC85h – xCFFh are available for the implementation of cardspecific configuration registers.

Configuration Bits Defined by EISA Spec

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