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ARM System Calls: Difference between revisions
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<pre>mov pc, lr</pre>
Most of the time you won't need to worry about returning from the interrupt, as GCC, set up to cross compile for ARM, lets you code the interrupt handlers in C:
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==Creating System Calls==
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b . @ FIQ
</pre>
This is the ARM equivalent to the IDT, on the x86, and it is stored by default at address 0. The only entry, we need to worry about is the SWI Handler. To install our own SWI handler, we replace the <span style="font-family:monospace">b .</span> instruction with a branch to our handler:
<pre>
interrupt_vector_table:
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</pre>
We can code the interrupt handler like this:
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void __attribute__ ((interrupt ("SWI"))) swi_handler (void) {}
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Parameters to functions on ARM, are passed in registers r0-r3, if follow the same convention for system calls, then our interrupt handler can take parameters:
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void __attribute__ ((interrupt ("SWI"))) swi_handler (int r0, int r1, int r2, int r3) {}
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You've probably noticed that <span style="font-family:monospace">swi</span> takes an integer argument. To get this argument in C, we have to do this:
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uint8_t int_vector = 0;
asm volatile ("ldrb %0, [lr, #-2]" : "=r" (int_vector));
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This loads the high 8 bits (16-23) of the argument into <span style="font-family:monospace">int_vector</span>, as loading the full 24-bits won't work using Thumb.
[[Category:ARM]]
[[Category:System Calls]]
[[Category:In Progress]]
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