CHAPTER 14. PROGRAMMABLE LED
The ZT 8905 includes two Light-Emitting Diodes (LEDs). A green LED for the optional IDE disk drive
is mounted at the bottom right corner of the board when viewed from the top with the gold fingers to
the left. A red general purpose LED is located just under the extractor. The red LED is software
programmable through the LED bit in System register 0 (see Chapter 11, "System Registers").
Writing a logical 0 to the LED bit turns the LED off and writing a logical 1 to the LED bit turns the LED
on. The LED is turned off after a power cycle or a reset.
The LED bit is in the same register as several system level functions. It is important to preserve the
state of the other bits in this register when modifying the LED status. The following code
demonstrates the mechanism for modifying the LED bit:
; set LED ON
cli
; clear interrupts
in al, 78h
; read current state
or al, 10h
; set LED bit
out 78h, al
; output new value for register
sti
; re-enable interrupts
; set LED OFF
cli
; clear interrupts
in al, 78h
; read current state
and al, 0EFh
; clear LED bit
out 78h, al
; output new value for register
sti
; re-enable interrupts
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