Specifications

CHAPTER 4

Software

ROM Software 39

One-Second Interrupt 4

As on the Macintosh Color Classic and Macintosh LC III computers, the one-second

interrupt on the Macintosh LC 520 computer is provided by the ADB microcontroller,

which sends the one-second interrupt to the main processor as a pseudodevice

transaction. In those cases where a one-second interrupt has been missed, the ADB

microcontroller sends the current value of the real-time clock so that the system software

can update the value stored in the Time global variable.

Pushbutton Interrupts 4

The ROM in the Macintosh LC 520 computer includes routines for initializing the

pushbutton interrupt bits in the interrupt enable and ﬂag registers and for initializing

other new registers that support the pushbuttons.

Pushing any of the four pushbuttons on the front of the case causes the machine to set a

bit in a new register, the Pushbutton register, which in turn causes a level-2 interrupt. The

interrupt handler disables the pushbutton interrupt until the button that caused the

interrupt is released.

Power Saver Software 4

The software that controls the brightness of the display also includes code that

implements the power saver mode, which turns off the power to the display after a set

interval of time. Applications can turn the monitor on and off and read its status by

making the appropriate call to the Screen driver (.BCScreen). For more information, see

the section “The Screen Driver” beginning on page 42.

Note

The screen can remain dark for several seconds after the screen is

reactivated, so the system emits a series of beeps to reassure the user that

the computer is still operating. The Screen driver call that turns

the monitor back on also causes a call to the Notiﬁcation Manager to play

the SysBeep sounds. When the screen is being reactivated, the keyboard

and mouse are disabled until the screen reappears. ◆

Video Software 4

Video support on the Macintosh LC 520 computer uses the same code as that used in the

Macintosh LC III computer. The only difference is that VRAM is always present in

the Macintosh LC 520.