ST20-C1 STMICROELECTRONICS [STMicroelectronics], ST20-C1 Datasheet - Page 76

• Manufacturer Part Number: ST20-C1
• Description: Instruction Set Reference Manual
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

6.7 Traps
6.7
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
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.
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ExceptionBase
(word offset)
Exception
Traps
Setting up the exception handler
level
Pointer OR 1
Exception
vector
table
Figure 6.2 Exception handler
or trapped state
ex.HandlerIptr
Interrupted
Exception
(7 words)
Handler
space
Work
Exception
handler code
entry point
Initial
exception handler
Wptr