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S3C2410A ARM INSTRUCTION SET
3-49
SOFTWARE INTERRUPT (SWI)
The instruction is only executed if the condition is true. The various conditions are defined in Table 3-2. The
instruction encoding is shown in Figure 3-24, below.
31 2427
1111Cond Comment Field (Ignored by Processor)
28 23
[31:28] Condition Field
0
Figure 3-24. Software Interrupt Instruction
The software interrupt instruction is used to enter Supervisor mode in a controlled manner. The instruction causes
the software interrupt trap to be taken, which effects the mode change. The PC is then forced to a fixed value (0x08)
and the CPSR is saved in SPSR_svc. If the SWI vector address is suitably protected (by external memory
management hardware) from modification by the user, a fully protected operating system may be constructed.
RETURN FROM THE SUPERVISOR
The PC is saved in R14_svc upon entering the software interrupt trap, with the PC adjusted to point to the word after
the SWI instruction. MOVS PC,R14_svc will return to the calling program and restore the CPSR.
Note that the link mechanism is not re-entrant, so if the supervisor code wishes to use software interrupts within
itself it must first save a copy of the return address and SPSR.
COMMENT FIELD
The bottom 24 bits of the instruction are ignored by the processor, and may be used to communicate information to
the supervisor code. For instance, the supervisor may look at this field and use it to index into an array of entry
points for routines which perform the various supervisor functions.
INSTRUCTION CYCLE TIMES
Software interrupt instructions take 2S + 1N incremental cycles to execute, where S and N are defined as sequential
(S-cycle) and non-sequential (N-cycle).