Specifications

CHAPTER 2
Architecture
Video Display Timing 25
Video RAM 2
The Macintosh LC 520 computer uses separate video RAM (VRAM) to store the screen
buffer. The video RAM occupies a dedicated address space, as shown in Figure 2-2.
The computer comes with 512 KB of VRAM soldered to the main logic board. A
68-pin SIMM socket accepts an additional 256 KB for a total of 768 KB of VRAM.
The VRAM SIMM is the same size and has the same pin assignments as the VRAM
expansion SIMM for the Macintosh Display Card 8•24. The system interface to the
VRAM is a 16-bit data bus using bits 31–16.
Note
The 256 KB VRAM SIMM used in the Macintosh LC 520 computer is the
same as that used in the Macintosh Color Classic; it is not the same as the
512 KB VRAM SIMM used in the Macintosh LC and Macintosh LC II
computers.
◆
A color lookup table (CLUT) provides color values for 4-bit and 8-bit video modes. With
the Monitors control panel set to black-and-white, the CLUT is still used, but all three
color components (R, G, and B) are the same.
Video Display Timing 2
The standard video display on the Macintosh LC 520 computer has the same number of
pixels as the display used with the AppleColor High-Resolution RGB Monitor: 640 by 480
pixels. Table 2-1 lists the video parameters; Figure 2-3 on page 26 shows the timing and
the synchronizing signals.
Table 2-1 Video parameters for the standard display
Parameter Value
Dot clock 30.24 MHz
Dot time 33.07 ns
Line rate 35.00 kHz
Line time 28.57 µs
Frame rate 66.67 Hz
Frame time 15.00 ms