Page 306 - Programming Microcontrollers in C

The MC68HC16 291

pointer. Finally the address 6 must contain the initial value of the IZ
register. Note that the word addresses in the MC68HC16 are always
even. In this machine you will find words on even boundaries, bytes
anywhere, and long words on even boundaries. These data are loaded
into this memory area by the use of a vector routine similar to that seen
in Chapter 5 for the MC68HC11. With this approach, all of the regis­
ters needed to begin operation of the basic computer are loaded from
memory at reset time. Of course, this operation does not eliminate the
need for program initialization; it merely provides a mechanism by
which the processor will start accessing memory at the correct ad­
dresses when the device comes out of reset.
Table 6-1 contains a listing of the uses of each entry in the
vector table. Note that vectors 0x0 through 0x37 have assigned
functions. The vectors 0x38 through 0xff are available for user-
defined operations. Note the relationship between the vector number
and the vector address. The vector address is always twice the vec-
Table 6-1: Exception Vector Table

Vector
Vector Address Type of Exception
Number
0 0x0000 Reset—Initial ZK, SK, PK
1 0x0002 Reset—Initial PC
2 0X0004 Reset—Initial SP
3 0X0006 Reset—Initial IZ
4 0X0008 Breakpoint
5 0X000A Bus Error
6 0X000C Software Interrupt
7 0X000E Illegal Instruction
8 0X0010 Division by Zero
9-E 0X0012-0X001C Unassigned, Reserved
F 0X001E Uninitialized Interrupt
10 0X0020 Level 0 Autovector
11 0X0022 Level 1 Autovector
12 0X0024 Level 2 Autovector
13 0X0026 Level 3 Autovector
14 0X0028 Level 4 Autovector
15 0X002A Level 5 Autovector
16 0X002C Level 6 Autovector
17 0X002E Level 7 Autovector
18 0X0030 Spurious Interrupt
19-37 0X0032-0X006E Unassigned, Reserved
38-FF 0X0070-0X01FE User-defined Interrupts