EISA PIO Bandwidth

EISA PIO Bandwidth

The EISA bus adapter is a device on the GIO bus. The GIO bus runs at either 25 MHz or 33 MHz, depending on the system model. Each EISA device access takes multiple GIO cycles, as follows:

The base time to do a native GIO read (of up to 64 bits) is 1 microsecond.
A 32-bit EISA slave read adds 15 GIO cycles to the base GIO read time; hence one EISA access takes 19 GIO cycles, best case.
A 4-byte access to a 16-bit EISA device requires 10 more GIO cycles to transfer the second 2-byte group; hence a 4-byte read to a 16-bit EISA slave requires 25 GIO cycles.
Each wait state inserted by the EISA device adds four GIO cycles.

Table 4-2 summarizes best-case (no EISA wait states) data rates for reading and writing a 32-bit EISA device, based on these considerations.

EISA Bus PIO Bandwidth (32-Bit Slave, 33-MHz GIO Clock)
Data Unit Size Read Write
1 byte 0.68 MB/sec 1.75 MB/sec
2 byte 1.38 MB/sec 3.51 MB/sec
4 bytes 2.76 MB/sec 7.02 MB/sec

EISA Bus PIO Bandwidth (32-Bit Slave, 33-MHz GIO Clock)
Data Unit Size	Read	Write
1 byte	0.68 MB/sec	1.75 MB/sec
2 byte	1.38 MB/sec	3.51 MB/sec
4 bytes	2.76 MB/sec	7.02 MB/sec

Table 4-3 summarizes the best-case (no wait state) data rates for reading and writing a 16-bit EISA device.

EISA Bus PIO Bandwidth (16-Bit Slave, 33-MHz GIO Clock)
Data Unit Size Read Write
1 byte 0.68 MB/sec 1.75 MB/sec
2 byte 1.38 MB/sec 3.51 MB/sec
4 bytes 2.29 MB/sec 4.59 MB/sec

EISA Bus PIO Bandwidth (16-Bit Slave, 33-MHz GIO Clock)
Data Unit Size	Read	Write
1 byte	0.68 MB/sec	1.75 MB/sec
2 byte	1.38 MB/sec	3.51 MB/sec
4 bytes	2.29 MB/sec	4.59 MB/sec