ST20-C1 datasheet(76/205 Pages) STMICROELECTRONICS

76 / 205 page

6.7 Traps

76/205

6.7

Traps

The ST20-C1 has a system of traps which act as software generated interrupts. Any

level of exception can be called as a user exception using

ecall. This mechanism can

be used for system calls to an operating system. In addition some special exception

levels are reserved for system use to provide for the trapping of breakpoints,

scheduling events, illegal operations and the machine becoming idle. The reserved

‘system’ exception levels are described in section 6.1.

If a system trap event occurs or a trap is called at an exception level which has a null

trap handler then the CPU ignores the trap and continues.

When a non-null trap handler is started, the trap_mode bit of the status register is set

and the user_mode bit is cleared. The interrupt_mode bit is not altered. The

trap_mode bit indicates that a trap handler is currently executing, so it is cleared if the

trap handler is interrupted.

6.8

Setting up the exception handler

To create an exception handler, an exception work space area must be created, with

enough space for the exception handler’s stack and 8 words at the top of the work

space for the exception control block, as defined in Table 6.3. For minimum interrupt

latency, interrupt handler control blocks should be in fast memory, preferably on-chip.

The normal work space and control block of an exception handler is shown in Figure

6.2. In the control block, ex.HandlerIptr must be initialized to point to the entry point of

the exception handler code.

Figure 6.2 Exception handler

ex.HandlerIptr

Pointer OR 1

Exception

handler code

entry point

Interrupted

or trapped state

Work

space

Exception

vector

table

Exception

level

(7 words)

(word offset)

ExceptionBase

Exception

Handler

Initial

exception handler

Wptr

ST20-C1 Datasheet(PDF) 76 Page - STMicroelectronics

ST20-C1 Datasheet(HTML) 76 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL