Software Compatible 6809 CPU

Memec Design

February 25, 2003

5

Optimized for

Interrupts

For interrupting the MC-ACT-6809, there are 3 level-sensitive interrupts:

irq_n:

standard interrupt or interrupt request, low_active, pushes all registers on stack:

PC(Low), PC(High), U(Low), U(High), Y(Low), Y(High), X(Low), X(High), DP, B, A, CC

firq_n: fast interrupt, pushes only PC and CC on stack: PC(Low), PC(High), CC

nmi_n: non maskable interrupt, pushes all registers on stack:

PC(Low), PC(High), U(Low), U(High), Y(Low), Y(High), X(Low), X(High), DP, B, A, CC

The nmi_n is masked after reset until the system stack pointer is initialized.

The different interrupts will fetch the corresponding vector for the interrupt in the memory:

0xfff0

Illegal Instruction

0xfff2

SWI3

0xfff4

SWI2

0xfff6

FIRQ

0xfff8

IRQ

0xfffa

SWI

0xfffc

NMI

0xfffe

RESET

The interrupt behavior of the MC-ACT-6809 matches exactly the one of the original 6809 in terms of results in

registers and memory. An exception is the case where an illegal instruction is decoded by the 6809: in this case, the

reserved interrupt is used.