MAS281 Dynex, MAS281 Datasheet - Page 14

• Manufacturer Part Number: MAS281
• Description: MIL-STD-1750A Microprocessor
• Manufacturer: Dynex

MAS281
register on the following SYNC high-to-low transition (with the
exception of INT02N which is latched into Pl when INT02N is
first detected). This sequence occurs whether interrupts are
enabled or disabled or whether the specific interrupt is masked
or unmasked.
detector to prevent repeat latching of requests held low
beyond the first SYNCN high-to-low transition. An interrupt
request input must transition to the high state before a
subsequent request on that input will be detected.
with its corresponding mask bit in the mask register (MK).
Interrupt level 0 is not maskable. Any unmasked pending
interrupts are output to the priority encoder where the highest
priority is encoded as a 4-bit vector. If interrupts are enabled,
and an unmasked interrupt is pending, the priority encoder will
assert an interrupt request to the CU.
instruction, the CU’s microsequencer checks the state of the
priority encoder’s interrupt request. If an interrupt request is
asserted, the microsequencer branches to the microcode
interrupt service routine. This routine performs a read of the
priority encoder’s 4-bit pending interrupt vector, stores the
value in the EU Dl register, and then uses this value to
calculate the appropriate interrupt linkage and service
pointers. The pointers serve as addresses to data structures
used in servicing interrupts. Figure 6 depicts this relationship.
Table 5 defines pointer values.
interrupt service routine performs the following: (1) the current
contents of the status word, mask register, and instruction
counter are saved; (2) a write status word l/O command is
executed with an all zero data word; (3) the new mask is
loaded into MK and interrupts are disabled; (4) the new status
word is read and checked for a valid AS field - If AS is non-zero
and an MMU is not present, AS is set to zero and fault 11
(address state error) is set in the fault register FT; (5) a write
status word command using the new status word is performed;
and (6) the new IC value is loaded into IC, the instruction
pipeline is filled starting at the new address, and instruction
execution begins.
actions performed during interrupt servicing and do not
necessarily reflect the actual order in which these events take
place.]
it will not be processed until interrupts are re-enabled. If an AS
fault occurs during the service routine, interrupt level 1 will be
set. This interrupt will be serviced when interrupts are re-
enabled unless it is masked by the new value in MK.
3.4 FAULT SERVICING
inputs will be latched into the Fault Register (FT) at the high-to-
low transition of SYNCN.
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Each external Pl register input is buffered by a falling edge
Upon completing execution of a given MIL-STD-1750A
If an interrupt is latched during the interrupt service routine,
Eight user fault inputs are provided. A low on any of these
When an interrupt request is latched into Pl, it is ANDed
[NOTE: The steps listed above represent a summary of
Using the linkage and service pointers, the microcode
PWRD
INT02
INT08
INT10
INT11
IOI1
INT13
IOI2
INT15
Table 5: Interrupt Pointer Definitions
Interrupt
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Figure 6: Interrupt Vectoring
LP
Address
20
22
24
26
28
2A
2C
2E
30
32
34
36
38
3A
3C
3E
SP
Address
21
23
25
27
29
2B
2D
2 F
31
33
35
37
39
3B
3D
3F