Apple Lisa - The CPU Board; The 68000; Interrupts

Confidential
Lisa Hardware Reference Manual
THE 68000
The 68000 is a 16 bit microprocessor with eight 32 bit data registers,
seven 32 bit address registers, a 32 bit program counter, and two 32
bit stack pointers. Its instruction set directly supports a wide
variety of data types and addressing modes. Its 24 bit address bus
provides a memory addressing range of 16 megabytes. The 68000 used
in the Lisa computer has a CPU clock period of 200 nanoseconds.
Since the memory cycle time is 800 nanoseconds, and since the
processor cannot access memory during video refresh cycles, enough
wait states are inserted to bring each instruction execution time to
a multiple of four cycles.
INTERRUPTS
7-Jul-81
The 68000 has a flexible interrupt handling structure. Interrupts are
generated by system errors, program errors, and external devices. When
the interrupt is detected, the 68000 enters the supervisor state and
begins executing instructions at the address provided by the exception
vector table. The source of the exception can provide the index into the
vector table, or the interrupt level can be used to provide the exception
vector ('autovectoring'). Internal Lisa devices all use autovectors.
External devices can use either option.
The interrupt system has seven fixed priority interrupts.
FIXED PRIORITY INTERRUPTS
Level
Type
7
Non-Haskable
Interrupt (highest priority)
6 lS-232 ports
5
Expansion slot 0
4
Expansion slot
1
3 bpansion slot
2
2 Keyboard Interrupt
1 All other internal
interrupts (lowest priority)
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