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A sidebar to the Apple Macintosh preview published in Byte, issue 2/1984, pp. 40.
The Macintosh Memory Map
The Macintosh memory map contains RAM, ROM, and I/O devices that communicate with the 68000 through specified memory locations. When the Macintosh is turned on (i.e., at boot-up), the 64K-byte ROM maps into the first page of memory and is used to get the system started. After boot-up, the positions of RAM and ROM are changed so that the 128K-byte block of RAM occupies the first two pages of memory (see figure below).
The Phase Read area of memory is used to determine whether the computer’s timing signals are correctly in phase with each other; this is usually done by ROM routines at boot-up.
The IWM locations are used by the Macintosh’s IWM (integrated Woz machine), which controls all access to the internal 3½-inch disk drive and the optional external one.
The SCC Read and SCC Write locations are used for several purposes. They allow the SCC (serial communications controller) chip to handle two serial ports at rates between 30 and 230,400 bits per second. In addition, they allow the SCC to detect mouse motion (in conjunction with the VIA) and adjust the phase of the Macintosh timing signals.
Most programmers will not need intimate knowledge of the Macintosh memory map. The 64K-byte ROM contains sophisticated routines that take care of low-level processes like I/O, memory management, video display, and similar tasks. Apple encourages the use of these routines; they mean less development time, conformity to the standard Macintosh user interface, faster programs (ROM always runs at full speed), and more memory space for programs and disks.
|Page added on 20th January 2004.
Copyright © 2002-2005 Marcin Wichary
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