Ibm SG24-4576-00 User Manual download pdf (Page 28)

1.5.1 ISA Bus

The Industry Standard Architecture (ISA) is not really an architecture at all but a

defacto standard based on the original IBM PC/AT bus design. The main

characteristics of the ISA bus include a 16-bit data bus and a 24-bit address bus.

The bus speed is limited to 8 MHz and it did not allow for DMA and bus masters

in its original form. It does not support automatically configuring adapters and

resolving resource conflicts among adapters nor does it allow for sharing of

interrupts. Nonetheless, it was an extremely successful design and even with

these disadvantages, it is estimated that the ISA bus is in 70% of the PCs

manufactured today.

1.5.2 EISA Bus

The Extended Industry Standard Bus Architecture (EISA) is a 32-bit superset of

the ISA bus providing improved functionality and greater data rates while

maintaining backward compatibility with the many ISA products already

available.

The main advancements of the EISA bus are 32-bit addressing and 16-bit data

transfer. It supports DMA and bus master devices. It is synchronized by an 8.33

MHz clock and can achieve data transfer of up to 33 MBps. A bus arbitration

scheme is also provided which allows efficient sharing of multiple EISA bus

devices. EISA systems can also automatically configure adapters.

1.5.3 Micro Channel Bus

The Micro Channel Architecture (MCA) was introduced by IBM in 1987. Micro

Channel is an improvement over ISA in all of the areas discussed in the previous

section on EISA. In addition, it supports data streaming which is an important

performance feature of the MCA architecture.

1.5.3.1 Data Streaming

The data streaming transfer offers considerably improved I/O performance. In

order to understand data streaming transfers we need to see how data is

transferred between Micro Channel bus master adapters and memory.

The standard method of transfer across the Micro Channel is known as basic

data transfer. In order to transfer a block of data in basic data transfer mode, an

address is generated on the address bus to specify where the data should be

stored; then the data is put on the data bus.

This process is repeated until the entire block of data has been transferred.

Figure 7 on page 14 shows basic data transfer in operation. Basic data transfer

on the Micro Channel runs at 20 MBps (each cycle takes 200 nanoseconds, and

32 bits or 4 bytes of data are transferred at a time).

Chapter 1. IBM PC Server Technologies 13

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Ibm SG24-4576-00 User Manual Page 28