T-Kernel Specification (1.B0.02)

186 CHAPTER 4. T-KERNEL/OS FUNCTIONS
When the TA ASM attribute is designated, in principle the OS is not involved in interrupt handler
starting. When an interrupt is raised, the interrupt handling function in the CPU hardware (depending
on the implementation, processing by T-Monitor may be included) directly starts the interrupt handler
defined by this system call. Accordingly, processing for saving and restoring registers used by the
interrupt handler is necessary at the beginning and end of the interrupt handler. An interrupt handler
is terminated by execution of the tk ret int system call or by the CPU interrupt return instruction
(or equivalent means).
Provision of a means for return from an interrupt handler without using tk ret int and without OS
intervention is mandatory. Note that if tk ret int is not used, delayed dispatching is not necessary.
Support for return from an interrupt handler using tk ret int is mandatory, and in this case delayed
dispatching is necessary.
When the TA HLNG attribute is designated, the interrupt handler is started via a high-level language
support routine. The high-level language support routine takes care of saving and restoring register
values. The interrupt handler terminates by a simple return from a function. The interrupt handler
takes the following format when the TA HLNG attribute is designated.
void inthdr( UINT dintno )
{
/*
Processing
*/
return; /* exit interrupt handler */
}
The parameter dintno passed to an interrupt handler is a number identifying the interrupt that was
raised, and is the same as that designated with tk def int. Depending on the implementation, other
information about the interrupt may be passed in addition to dintno. If such information is used, it
must be defined for each implementation in a second parameter or subsequent parameters passed to the
interrupt handler.
If the TA HLNG attribute is designated, it is assumed that the CPU interrupt flag will be set to interrupts
disabled state from the time the interrupt is raised until the interrupt handler is called. In other words,
as soon as an interrupt is raised, the state goes to multiple interrupts disabled, and this state remains
when the interrupt handler is called. If multiple interrupts are enabled, the interrupt handler must
include processing that enables interrupts by manipulating the CPU interrupt flag.
Also in the case of the TA HLNG attribute, upon entry into the interrupt handler, system call issuing
must be possible. Note, however, that assuming standard provision of the functionality described above,
extensions are allowable such as adding a function for entering an interrupt handler with multiple
interrupts enabled.
When the TA ASM attribute is designated, the state upon entry into the interrupt handler is defined
for each implementation. Such matters as the stack and register status upon interrupt handler entry,
whether system calls can be made, the method of invoking system calls, and the method of returning
from the interrupt handler without OS intervention must all be defined explicitly.
In the case of the TA ASM attribute, depending on the implementation there may be cases where interrupt
handler execution is not considered to be a task-independent portion. In such a case the following points
need to be noted carefully.
• If interrupts are enabled, there is a possibility that task dispatching will occur.
• When a system call is invoked, it will be processed as having been called from a task portion or
quasi-task portion.
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T-Kernel 1.B0.02