Motorola M68CPU32BUG User Manual download pdf (Page 32)

MOTOROLA MC68332TUT/D

4.1.3.1 Initializing the Reset Vector

Immediately after the release of RESET, an internal state machine fetches the word values at addresses

$000000 through $000006 and loads them into the stack pointer and program counter. These values, gen-

erally referred to collectively as the as reset vector, are shown in Table 6. The values in the reset vector

must be initialized in order for program execution to begin.

Sample code to initialize the reset vector follows. Make sure that the stack does not overlap the program

code (the stack grows downward in memory). If the assembler does not recognize “DW”, check the manual

to determine the format for defining a constant word. Another common format is “DC.W.”

org $0000 ;begin at address $000000 of memory map

DW $0000 ;initial stack pointer = $4000

DW $4000

DW $0000 ;initial PC = $400

DW $0400

4.1.3.2 Initializing Exception Vectors other than Reset

Each exception vector should point to a handler routine in case the exception is accidentally taken. In an

actual program, the vectors would point to different labels, but in the example below, all of the vectors point

to the same label (INT). This label must be included later on in the code in case an exception is taken. This

example assumes that the label INT is at an address less than $10000. In other words, the upper word of

the address is $0000.

Table 5 Exception Vector Table

Vector Number

(Decimal)

Vector Offset

(Hexadecimal)

Assignment

0 0 Reset: Initial Stack Pointer

1 4 Reset: Initial Program Counter

2 - 15 8 - 3C Various Errors and Exceptions

16 - 23 40 - 5C Unassigned, Reserved

24 60 Spurious Interrupt

25 64 Level 1 Interrupt Autovector

26 68 Level 2 Interrupt Autovector

27 6C Level 3 Interrupt Autovector

28 70 Level 4 Interrupt Autovector

29 74 Level 5 Interrupt Autovector

30 78 Level 6 Interrupt Autovector

31 7C Level 7 Interrupt Autovector

32 - 47 80 - BC Trap Instruction Vectors

48 - 63 C0 - FC Unassigned/Reserved

64 - 255 100 - 3FC User Defined Vectors

Table 6 Reset Vector for CPU32

Address Reset Vector

$0000 Initial Stack Pointer Upper Word

$0002 Initial Stack Pointer Lower Word

$0004 Initial Program Counter Upper Word

$0006 Initial Program Counter Lower Word

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 55 56

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