AT&T 3B2/300 Technical Reference Manual page 478

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INTEL 80186 Microprocessor
The intelligence of the ISC card is provided by an INTEL 80186, 16-bit microprocessor operating at
8 MHz. Some of the microprocessor features used for the ISC card are described in the following
paragraphs. Figure 3-70 shows the ISC card address map.
DMA Channels
Direct Memory Access (DMA) Channel O is used by the CIO firmware to move data between the
ISC card RAM and the 3B2 computer main memory. DMA Channel 1 is programmed to do memory
refresh operations for the ISC internal RAM (along with Timer 2).
Interrupt Controller
The internal Interrupt Controller of the 80186 Microprocessor is programmed to accept five separate
interrupts: INT3 through INTO, and NMI. Interrupts INTO and INTl are reserved for CIO firmware
use. INT2 is dedicated to the 8274 Dual Channel Communications Chip. INT3 is dedicated to the
8237 DMA controller. Nonmaskable Interrupt (NMI) is used by the CIO firmware to control the Bus
Abort Feature (BAF).
Internal Timers
Timer O is available to applications software since external hardware is used to generate the 1/0
bus PFAILO signal. Timer 1 controls the BAF. Timer 2 is used to generate DMA requests to DMA
Channel 1 for starting memory refresh operations.
Memory and Peripheral Chip Selects
The memory and peripheral chip selects are programmed by the ISC firmware to provide chip
selects in the 80186 Microprocessor memory map (Figure 3-70).
Internal Clock Generator
The 80186 Microprocessor operates at 8 MHz (crystal controlled). The 80186 Microprocessor
provides an 8-MHz output for other ISC functions. The 8-MHz signal is divided to provide 4 MHz to
the 8237 DMA Controller and the 8274 Dual Channel Communications Chip.
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