1.4 KiB
"BBU" Apple Custom Silicon
The "BBU", as it is called on the Macintosh SE's printed circuit board silkscreen, is a relatively complex Apple custom silicon chip, compared to the other custom chips on the Macintosh SE's Main Logic Board (MLB). Despite its intimidating look as a chip with a huge number of pins, its purpose can be summarized as follows.
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Provide a single address bus interface to ROM, RAM, and I/O devices, including simple digital I/O pins.
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Scan the CRT by driving the primary digital control signals (
*VSYNC,*HSYNC,VIDOUT).
There might be additional processing functions it may provide as a convenience between the CPU and the various other hardware chips, but chances are these processing functions are relatively simple.
Most of the I/O pins that are connected to the BBU are single-bit digital I/O signals that are relatively easy to understand. Reverse engineering the Macintosh SE's firmware may be required to determine how these pins are mapped into the CPU's address space, but once that determination is made, providing a replica interface to most of the connected hardware should be super-easy.
The following I/O chips are connected to the BBU:
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VIA interrupt controller
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IWM/SWIM floppy disk controller
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SCSI Controller
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Serial Communications Controller (SCC)
Other chips that are connected to the BBU are mainly interfaced via only simple, single-pin interfaces.