Specifications

CHAPTER 2
Architecture
20 Block Diagram
This chapter describes the architecture of the Macintosh LC 520 computer. It describes
the main components on the logic board and explains the features that are different from
those of earlier Macintosh computers.
Block Diagram 2
The architecture of the Macintosh LC 520 computer is based on the design of the
Macintosh LC III. The Macintosh LC 520 uses several new custom ICs, as shown
in the block diagram in Figure 2-1.
MC68030 Microprocessor 2
The Macintosh LC 520 computer uses a Motorola MC68030 microprocessor running at a
clock speed of 25 MHz.
The Macintosh LC 520 computer does not have a built-in floating-point unit (FPU). The
main circuit board has a socket for adding an FPU. Also, an expansion board can provide
an FPU, because the /FPU select signal is available on the expansion connector. For more
information, see Chapter 3, “Expansion.”
Ardbeg Custom IC 2
A custom IC called Ardbeg combines several functions performed by individual ICs in
older machines:
■ timing and clock generation
■ memory mapping
■ VIA1 and VIA2 registers
■ video addressing and timing
■ SWIM II floppy disk interface
■ sound timing and control
■ interface to front-panel pushbuttons
The VIA functions are similar to those in the Macintosh LC III. The first VIA is a
full-function VIA, and the second is a set of registers like those in the V8 custom IC.
Ariel Custom IC 2
Another custom IC, called Ariel, contains the video CLUT (color look-up table) and DAC.
The Ariel IC is pin and software compatible with the AC843 but does not support 24 bits
per pixel. For information about the number of bits per pixel provided on different video
monitors, see the section “Video RAM” on page 25.