The ARM-THUMB Procedure Call Standard

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The ARM-THUMB Procedure Call StandardNoteNoteIn the limit-checked variants of this standard, SL (r10) is neither preserved nor altered by the calledroutine itself, but only by limit-checking support code.Return is usually to the instruction following the call sequence, but this standard does not require that.A called routine need not preserve the values of r0-r3, IP (r12) and LR (r14). Formally:Pre: stack limit
The ARM-THUMB Procedure Call Standard4.4.1 Variable number of parameters (variadic routines)The list of machine parameter values is converted to integer machine words as if by storing each value in turn intoconsecutive memory words. These integer parameter words are passed to a variadic routine as if by:o Loading the first 4 words into integer registers a1-a4 (lowest addressed into a1). If there are fewer than 4words, they are loaded into a1-a3, a1-a2, or a1 only.oNotePushing remaining words onto the stack in reverse order (so the first remaining parameter word is atVAL(SP), the second at VAL(SP)+4, and so on).Consequently, a floating-point value can be passed in integer registers, or can be split between aninteger register and memory.4.4.2 Fixed number of parametersMachine-level parameter values are passed to a non-variadic routine as if:oooNoteThe first N floating-point values are removed from the parameter list and assigned to floating-point argumentregisters of the appropriate precision. The number N and the details of the assignment depend on theselected floating-point architecture (see section 4.6, The FPA procedure call standard and 4.7, The VFP(scalar mode) procedure call standard).The first 4 integer values are removed from the parameter list and assigned to integer registers a1-a4. Fewerthan 4 integer parameter values there are assigned to a1-a3, a1-a2, or a1 only.Remaining values are pushed onto the stack in reverse order.Consequently, a machine-level floating-point value is passed in a floating-point register or in memory,never in integer registers. (But, a source-level floating-point field does not always map to a machinelevelfloating-point value. Consider, for example, a floating-point field in an inhomogeneous structure.)4.5 Result returnA procedure returns no result.A function returns a single value of integer sort or floating-point sort. A value occupies 1 or more words.A function of integer sort:o Must return a 1-word value in a1.ooMay return a value of length 2-4 words in a1-a2, a1-a3, or a1-a4, respectively.Must return a longer value indirectly, in memory, via an additional address parameter.A function of floating-point sort:ooMust return an atomic floating-point value in f0 or d0, as appropriate to its precision.May return a compound floating-point value in f0-fN, or d0-dN, where N depends on the selected floating-pointarchitecture (see section 4.6, The FPA procedure call standard and 4.7, The VFP (scalar mode) procedurecall standard).Section 7, ARM C and C++ Conventions, describes how source language entities of various lengths are returned.SWS ESPC 0002 A-05 Page 13 of 37
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The ARM-THUMB Procedure Call Standard

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