Theory of Operation
a unique vector. If STD DOS is installed, this number decreases
depending upon the number of devices in the system.
Non-Maskable Interrupts
In addition to the eight interrupt inputs at the interrupt controller, the
ZT 8809A supports three sources of interrupt referred to as "non-
maskable" interrupts (NMI). This type of interrupt has higher priority
over any of the maskable interrupts from the interrupt controller and is
not software maskable. Two of these sources may be disabled by
jumpers on board.
The three sources of interrupt are:
1.
STD bus NMIRQ*
2.
AC Power-Fail
3.
8087 Numeric Data Processor interrupt
These sources are logically ORed together and routed to the CPU. AC
power-fail non-maskable interrupt request is disabled by removal of
jumper W1. This source of interrupt should be enabled only if the
optional AC converter is attached to connector J5, which allows
detection of AC power to the STD bus system power supply. The
8087 Numeric Data Processor interrupt request is disabled by
removing W51 and installing W52. This interrupt request should be
enabled only if the zSBC 337 option for mounting the 8087 to the
ZT 8809A is attached.
3-14
