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Chapter 5 Input/Output Interfaces
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition – September 1997
5-26
FIFO Register, I/O Port 7F8h (ECP Mode Only)
While in ECP/forward mode, this location is used for filling the 16-byte FIFO with data bytes.
Reads have no effect (except when used in Test mode). While in ECP/backward mode, reads
yield data bytes from the FIFO.
Configuration Register A, I/O Port 7F8h (ECP Mode Only)
A read of this location yields 10h, while writes have no effect.
Configuration Register B, I/O Port 7F9h (ECP Mode, Read Only)
A read of this location yields the status defined as follows:
Bit Function
7 Reserved (always 0)
6 Status of Selected IRQ
n
.
5,4 Selected IRQ Indicator:
00 = IRQ7
11 = IRQ5
All other values invalid.
3 Reserved (always 1)
2..0 Reserved (always 000)
Extended Control Register B, I/O Port 7FAh (ECP ModeOnly)
This register defines the ECP mode functions.
Bit Function
7..5 ECP Submode Select:
000 = Standard forward mode (37Ah <5> forced to 0). Writes are
controlled by software and FIFO is reset.
001 = PS/2 mode. Reads and writes are software controlled and
FIFO is reset.
010 = Parallel Port FIFO forward mode (37Ah <5> forced to 0). Writes are
hardware controlled.
011 = ECP FIFO mode. Direction determined by 37Ah, <5>. Reads and
writes are hardware controlled.
4 ECP Interrupt Mask:
0 = Interrupt is generated on ERR- assertion.
1 = Interrupt is inhibited.
3 ECP DMA Enable/Disable.
0 = Disabled
1 = Enabled
2 ECP Interrupt Generation with DMA
0 = Enabled
1 = Disabled
1 FIFO Full Status (Read Only)
0 = Not full (at least 1 empty byte
1 = Full
0 FIFO Empty Status (Read Only)
0 = Not empty (contains at least 1 byte)
1 = Empty