Extern | ARM VERSION 1.2 specification

Directives Reference

7.7.9EXTERN

The EXTERN directive provides the assembler with a name that is not defined in the current assembly.

EXTERN is very similar to IMPORT, except that the name is not imported if no reference to it is found in the current assembly (see IMPORT on page 7-62, and EXPORT or GLOBAL on page 7-58).

Syntax

EXTERN symbol{[WEAK]}

where:

symbol is a symbol name defined in a separately assembled source file, object file, or library. The symbol name is case-sensitive.

[WEAK] prevents the linker generating an error message if the symbol is not defined elsewhere. It also prevents the linker searching libraries that are not already included.

Usage

The name is resolved at link time to a symbol defined in a separate object file. The symbol is treated as a program address. If [WEAK] is not specified, the linker generates an error if no corresponding symbol is found at link time.

If [WEAK] is specified and no corresponding symbol is found at link time:

•If the reference is the destination of a B or BL instruction, the value of the symbol is taken as the address of the following instruction. This makes the B or BL instruction effectively a NOP.

•Otherwise, the value of the symbol is taken as zero.

Example

This example tests to see if the C++ library has been linked, and branches conditionally on the result.

AREA	Example, CODE, READONLY
EXTERN	__CPP_INITIALIZE[WEAK]	; If C++	library linked, gets the address of
		; __CPP_INITIALIZE function.
LDR	r0,__CPP_INITIALIZE	; If not	linked, address is zeroed.
CMP	r0,#0	; Test if zero.
BEQ	nocplusplus	; Branch	on the result.

7-60

Copyright © 2000, 2001 ARM Limited. All rights reserved.

ARM DUI 0068B

Image 342

ARM VERSION 1.2 manual Extern

Contents

ARM Developer Suite ARM Developer Suite Assembler GuideCopyright 2000, 2001 ARM Limited. All rights reserved ARM Developer Suite Assembler Guide Glossary About this book on Feedback on Intended audience Using this bookAbout this book This book is organized into the following chapters Further reading Typographical conventionsItalic ARM publications ARM Reference Peripheral Specification ARM DDI Other publications Feedback Feedback on the ARM Developer SuiteFeedback on this book ARM DUI 0068B Introduction About the ARM Developer Suite assemblers on About the ARM Developer Suite assemblers ARM Developer Suite ADS has Writing ARM and Thumb Assembly Language Introduction Code examplesOr run the module in AXD with interleaving on See ARM and Thumb state Overview of the ARM architectureArchitecture versions ARM processors always start executing code in ARM state Processor mode RegistersGeneral-purpose, 32-bit registers Program counter pc ARM instruction set overview Branch instructionsData processing instructions Multiple register load and store instructions Status register access instructionsSingle register load and store instructions Semaphore instructions ARM instruction capabilities Register accessAccess to the inline barrel shifter Following general points apply to ARM instructions Thumb instruction set overview Thumb instruction capabilitiesFollowing general points apply to Thumb instructions Access to the barrel shifter Differences between Thumb and ARM instruction sets Single register load and store instructions General form of source lines in assembly language is Structure of assembly language modulesLayout of assembly language source files Case rules Labels Local labelsComments Constants Is a base between 2 Xxx is a number in that base An example ARM assembly language module ELF sections and the Area directive Application termination Entry directiveApplication execution END directive Calling subroutines An example Thumb assembly language module CODE32 and CODE16 directivesBX instruction Using the C preprocessor Example 2-4 Preprocessing an assembly language source file Conditional execution ALU status flags Execution conditions Examples Using conditional execution in ARM state Example of the use of conditional execution Conditional branches only Converting to Thumb Branch prediction and caches Loading constants into registers Direct loading with MOV and MVN Right, 2 bits Direct loading with MOV in Thumb state Loading with LDR Rd, =const Placing literal pools Loading floating-point constants Loading addresses into registers Direct loading with ADR and Adrl ADR Implementing a jump table with ADR Example 2-7 ARM code jump table Example 2-8 Thumb code jump table Loading addresses with LDR Rd, = label R1, =Darea + = LDR R1,PC, #offset into Literal Pool An LDR Rd, =label example string copying Address registers. For example, the instructionIncrements r1 by Load register Load and store multiple register instructions Syntax of the LDM instructions is ARM LDM and STM instructionsSyntax Where Usage LDM and STM addressing modes Implementing stacks with LDM and STM Descending or ascending Stacking registers for nested subroutines Block copy with LDM and STM Example 2-11 Block copy Movs Push and POP Thumb LDM and STM instructionsLDM and STM Thumb-state block copy example LSR This macro can be invoked as follows Using macrosTest-and-branch macro example After substitution this becomes After the instructions are executed, it holds the remainder Unsigned integer division macro exampleRegister that holds the divisor If only the remainder is required Ratio DivMod r0,r5,r4,r2 Describing data structures with MAP and Field directives Relative maps Register-based maps Program-relative maps Finding the end of the allocated data Forcing correct alignment EndOfChars Using register-based MAP and Field directives Defining register-based symbols Setting up a C-type structure Is equivalent to the C code Making faster access possible If you want the equivalent of the C code2-27 for an explanation of these This example, the MAP directive is Not Field MAP Using two register-based structures Avoiding problems with MAP and Field directives ArrayBase RN r9 Using frame directives Symbols on Expressions, literals, and operators on Command syntax Specifies that the content of inputfile is read-write Specifies that the content of inputfile is read-onlyPosition-independent. The default is /noropi Position-independent. The default is /norwpi Valid options are Assembled for the wrong target FPUSelection of libraries, accordingly Selects no floating-point option. This makes your assembled Object file, for use by the debugger see Keep on Command-line optionsSets the maximum source cache size to n. The default is 8MB Allow unaligned LDRs Register names First pass and reads them from memory on the second passAs \n and \t Turns off warning messages Assembler Reference Format of source lines Predeclared program status register names Predefined register and coprocessor namesPredeclared register names Predeclared floating-point register names Built-in variables Lists the built-in variables defined by the ARM assemblerExpressions or conditions, for example Determining the armasm version at assembly time Numeric constants on SymbolsSymbol naming rules Labels on Variables Numeric constants Assembly time substitution of variables Gbls DCQ and Dcqu on DCW and Dcwu on Program-relative labelsDCD and Dcdu on Dcfd and Dcfdu on Dcfs and Dcfsu on Register-relative labels Syntax of a local label is Syntax of a reference to a local label is Assembler Reference Expressions, literals, and operators This section contains the following subsections Example String expressionsString literals String literals Numeric expressions Numeric literals can take any of the following forms Is a sequence of characters using only the digits 0 to nNumeric literals Numeric code of the character Floating-point literals Floating-point literals can take any of the following forms Logical literals Register-relative and program-relative expressionsLogical expressions There are only two logical literals Operator precedence String manipulation Operator precedence in C Precedence Unary operators Operator Usage Description Example of use of SBOFFSET1912 and SBOFFSET11 Binary operators Multiplicative operatorsString manipulation operators Shift operators SHR is a logical shift and does not propagate the sign bitAddition, subtraction, and logical operators Relational operators Boolean operators 10 shows the Boolean operators ARM DUI 0068B ARM Instruction Reference Add with carry, Add All Logical Branch Move not All ARM condition codes Q flag LDR and STR, halfwords and signed bytes on ARM memory access instructionsLDR and STR, words and unsigned bytes on LDR and STR, doublewords on Where Is either LDR Load Register or STR Store Register LDR and STR, words and unsigned bytesOtherwise, a 32-bit word is transferred Program-relative Zero offsetPre-indexed offset Post-indexed offset Flexible offset syntax Address alignment for word transfers Loading to r15 Saving from r15 Architectures LDR and STR, halfwords and signed bytes Offset syntax Must be within ±255 bytes of the current instructionIs an offset applied to the value in Rn see Offset syntax Is often a numeric constant see examples below You cannot load halfwords or bytes to r15 Offset syntax is the same for LDR and STR, doublewords onAddress alignment for halfword transfers Incorrect example Must be an even numbered register, and not r14 LDR and STR, doublewordsIs an optional condition code see Conditional execution on Pre-indexed without writeback Not be the same as Rd or Rd+1 Address alignment Incorrect examples Increment address after each transfer Is either LDM or STMIs any one of the following Increment address before each transfer Non word-aligned addresses Loading or storing the base register, with writeback 5 PLD Is the register on which the memory address is based Alignment Is swapped with the contents of the memory location 6 SWPBoth Rd and Rm ARM general data processing instructions Flexible second operand LSR and LSL Bits of the register are set to Carry flag Instruction substitution Is the ARM register for the result 2 ADD, SUB, RSB, ADC, SBC, and RSCIs one of ADD, SUB, RSB, ADC, SBC, or RSC Is the ARM register holding the first operand Condition flags Use of r15Multiword arithmetic examples These instructions subtract one 96-bit integer from another 3 AND, ORR, EOR, and BIC Logical AND, OR, Exclusive or and Bit ClearIs one of AND, ORR, EOR, or BIC Orreq MOV and MVN Move and Move NotIs the ARM register for the result Mvnne CMP and CMN Compare and Compare Negative CMN TST and TEQ Test and Test Equivalence TEQ Is the ARM register for the result. Rd must not be r15 7 CLZCount Leading Zeroes Is the operand register ARM multiply instructions MUL and MLA onUMULL, UMLAL, Smull and Smlal on MUL and MLA MUL UMULL, UMLAL, Smull and Smlal Is one of UMULL, UMLAL, SMULL, or Smlal Umull Use the top end bits 3116 of Rs SMULxyUse the top end bits 3116 of Rm Are the ARM registers holding the values to be multiplied Smulbt SMLAxy Is the ARM register holding the value to be added Smlatb SMULWy Use the top end bits 3116 of RsAre the ARM registers holding the operands SMLAWy Are the ARM registers holding the values to be multipliedIs the ARM register holding the value to be added Smlawt SMLALxy Smlaltt Use the top end bits 3116 of Rm 8 MIA, MIAPH, and MIAxyCurrent processors R15 cannot be used for either Rm or Rs These instructions are only available in XScale Is one of QADD, QSUB, QDADD, or Qdsub ARM saturating arithmetic instructionsQADD, QSUB, QDADD, and Qdsub Are the ARM registers holding the operands Qadd ARM branch instructions Branch, and Branch with Link 2 BX 3 BLX Blxmi ARM coprocessor instructions MCR, MCR2, Mcrr on 1 CDP, CDP2 Is p n, where n is an integer in the range 2 MCR, MCR2, McrrAre ARM source registers. They must not be r15 3 MRC, MRC2 Affected Mrrc Is an optional suffix specifying a long transfer 5 LDC, STCIs either LDC or STC Is the coprocessor register to load or save Architectures 6 LDC2, STC2 Is either LDC2 or STC2 Architectures Miscellaneous ARM instructions 1 SWI Software interrupt Is the destination register. Rd must not be r15 2 MRSWhere Is either Cpsr or Spsr 3 MSR Is either Cpsr or SpsrSee MRS on MSR CPSRf, r5 Bkpt Breakpoint 5 MAR, MRA For current processorsAre general-purpose registers Adrl ARM pseudo-instructionon ARM pseudo-instructionsADR ARM pseudo-instructionon LDR ARM pseudo-instruction on Is the register to load ADR ARM pseudo-instructionIs an optional condition code Non word-aligned address within ±255 bytes Adrl ARM pseudo-instruction Non word-aligned address within 64KBWord-aligned address within 256KB R4,start + = ADD R4,pc,#0xe800 LDR ARM pseudo-instruction R3,=0xff0 Loads Into = MOV r3,#0xff0 NOP ARM pseudo-instruction Thumb Instruction Reference Add with carry Rotate right LDR and STR, register offset on Thumb memory access instructionsLDR and STR, immediate offset on LDR and STR, pc or sp relative on Load register Where Is eitherLDR and STR, immediate offset Store register Address alignment for word and halfword transfers LDR and STR, register offset Strsh LDR and STR, pc or sp relative 4 in the range 0 to R2,pc,#1016 POP reglist POP reglist, pc These instructions do not affect the flags Load multiple, increment after Ldmia and StmiaLoad and store multiple registers Store multiple, increment after R3!, r0,r4 ADD, pc or sp relative on Thumb arithmetic instructionsADD and SUB, sp on ADC, SBC, and MUL on ADD and SUB, low registers To +7255 to +255 These instructions update the N, Z, C, and V flags Restrictions 2 ADD, high or low registers Is a register containing the second operand ADD and SUB, sp Range -508 to +508 Is either sp or pc 4 ADD, pc or sp relativeIs the destination register. Rd must be in the range r0- r7 Range Where Is one of ADC, SBC, or MUL 5 ADC, SBC, and MUL MOV, MVN, and NEG on page 5-28 Move, Move NOT, and Negate Thumb general data processing instructionsCMP and CMN on page 5-26 Compare and Compare Negative TST on page 5-30 Test bits Bitwise logical operations Where Is one of AND, ORR, EOR, or BIC1 AND, ORR, EOR, and BIC Range r0- r7 2 ASR, LSL, LSR, and ROR Register-controlled shift Immediate shift Where Is the register containing the first operand Examples Move, Move NOT, and Negate Where Is the destination register4 MOV, MVN, and NEG Is the source register Condition flags Test bits Where Is the register containing the first operand5 TST Rn and Rm must be in the range r0-r7 Thumb branch instructions 3-23 for more information 1 BIs an optional condition code see -2 on Label must be within Condition codes for Thumb B instruction 2 BL Long branch with Link 3 BX Instruction clears the T flag in the CPSR. Code at 4 BLXBranch with Link, and optionally exchange instruction set BLX label always causes a change to ARM state Thumb software interrupt and breakpoint instructions Bkpt on Bkpt immed8 Thumb pseudo-instructions ADR Thumb pseudo-instruction 1KB. expr must be defined locally, it cannot be imported LDR Thumb pseudo-instruction If the value of expr is within range of a MOV instruction,Assembler generates the instruction =labelname NOP Thumb pseudo-instruction Syntax for NOP is ARM DUI 0068B Vector Floating-point Programming Absolute value Vector All Negate Vector All Vector floating-point coprocessor Reference ManualVFP architectures Floating-point registers Register banks Vector stride VectorsVector wrap-around Restriction on vector length Scalar operations Vector and scalar operationsControl of scalar, vector and mixed operations Vector operations VFP and condition codes Vector Floating-point Programming VFP system registers FPSCR, the floating-point status and control register 0b000 Modifying individual bits of a VFP system register FPEXC, the floating-point exception registerFPSID, the floating-point system ID register See FMRX, FMXR, and Fmstat on Flush-to-zero mode When to use flush-to-zero modeEffects of using flush-to-zero mode Operations not affected by flush-to-zero mode FMRX, FMXR, and Fmstat on VFP instructionsFmrrs and Fmsrr on Ftosi and Ftoui onPage Fabsd d3, d5 Fnegsmi s15, s15 Is the VFP register holding the first operand Fadd and FsubIs the VFP register for the result Is the VFP register holding the second operand With zero instruction FcmpFloating-point compare Fcmp is always scalar Fcmp instructions can produce Invalid Operation exceptions Fcvtds Is a double-precision VFP register for the resultIs a single-precision VFP register holding the operand Fcvtsd Is a single-precision VFP register for the resultIs a double-precision VFP register holding the operand Fdiv Precision specified in precision FLD and FSTFloating-point load and store Address used for the transfer Fldsne Fldm and Fstm Unspecified precision Following instructions are equivalent FMAC, FNMAC, FMSC, and Fnmsc Must be one of FMAC, FNMAC, FMSC, or Fnmsc Fnmscsle Are ARM registers. Do not use r15 Fmdrr and FmrrdIs the VFP double-precision register These instructions do not produce any exceptions FMDHR, FMDLR, FMRDH, and Fmrdl Is the ARM register. Rd must not be r15These instructions are used together as matched pairs Fmrs and Fmsr Is the VFP single-precision register Fmrrs and Fmsrr Are two consecutive VFP single-precision registersAre the ARM registers. Do not use r15 FMRX, FMXR, and Fmstat Is the ARM register Fmul and Fnmul Fsito and Fuito Fsqrt Is the VFP register holding the operand Ftosi and Ftoui Is a single-precision VFP register for the integer result FLD pseudo-instruction VFP pseudo-instructionThere is one VFP pseudo-instruction Can be S for single-precision, or D for double-precision D1,=3.12E106 Loads 3.12E106 into d1 VFP directives and vector notation Vfpassert Scalar on Vfpassert Vector on Vfpassert Scalar VFP directives and vector notation on Vfpassert Vector on Where Is the vector length Is the vector stride VFP directives and vector notation on Vfpassert Scalar on R10,FPSCR ARM DUI 0068B Assembly control directives on Conditional assembly, looping, inclusions, and macros Alphabetical list of directives Location of descriptions of directives GBLA, GBLL, and Gbls on Symbol definition directivesThis section describes the following directives Declare a global arithmetic, logical, or string variable GBLA, GBLL, and Gbls Armasm -pd objectsize Seta 0xFF -o objectfile sourcefile LCLA, LCLL, and Lcls SETA, SETL, and Sets Rlist Names on Coprocessor names on 5 CNCN directive defines a name for a coprocessor register Evaluates to a coprocessor register number from 0 to 6 CP Evaluates to a coprocessor number from 0 to DN and SN 8 FN Evaluates to a floating-point register number from 0 to Data definition directives DCD and Dcdu onDCQ and Dcqu on Ltorg Set to this address 2 MAPIs a numeric or program-relative expression During the first pass of the assembler Field By the value of exprStorage counter Space Expressions on Quoted string. The characters of the string are loaded into 5 DCBTo 255 see Numeric expressions on Consecutive bytes of store Program-relative expression DCD and DcduNumeric expression see Numeric expressions on DCW and Dcwu on DCQ and Dcqu on Dcdo Dcfd and Dcfdu Dcfs and Dcfsu 10 DCI Is a numeric expression see Numeric expressions on DCQ and Dcqu DCD and Dcdu on DCW and Dcwu on DCD and Dcdu on DCQ and Dcqu on DCW and DcwuTo 65535 see Numeric expressions on Data Mexit on Assembly control directivesMacro and Mend on IF, ELSE, and Endif on While and Wend on Macro and Mend BGE Using a macro to produce assembly-time diagnostics Mexit 4 IF, ELSE, and Endif See Relational operators on Example 7-3 Assembly conditional on a variable being defined While and Wend Frame Address on Frame Register on Frame Restore onFrame description directives Frame POP on Frame Push on Is sp unless the function uses a separate frame pointer Can omit it There are two alternative syntaxes for Frame POP Is the number of bytes that the stack pointer moves Frame Push There are two alternative syntaxes for Frame Push Frame Register Is the register in which the value is preserved Frame Restore Frame Save Frame State Restore on Function or Proc on Frame State Remember Frame State Restore Frame State Remember on Function or Proc on Function or Proc Endfunc or Endp Reporting directives AssertIs an assertion that can evaluate to either True or False Info Is an expression that evaluates to a string Where Is the OPT directive setting. -2 lists valid settings 3 OPTSpecify the -listassembler option to turn on listing OPT TTL and Subt Is the titleIs the subtitle Export or Global on Miscellaneous directivesCODE16 and CODE32 on GET or Include on Align Cacheable, CODE, ALIGN=3 Area Example,CODE,READONLY An example code section CODE16 and CODE32 4 END Entry Entry directive declares an entry point to a program Address, or a 32-bit integer constant Is the symbolic name to assign to the value6 EQU Is optional. type can be any one Export or Global Exportas Extern GET or Include Nesting directives on See Export or Global on File, or library. The symbol name is case-sensitiveGlobal Import Incbin IncludeSee GET or Include on Symbols are kept except register-relative symbols Keep Require Assembly failsNofp Require directive specifies a dependency between sections REQUIRE8 and PRESERVE8 19 RN Evaluates to a register number from 0 to Rout Thumb state See ARM Developer SuiteAmong other things, computer software Platforms See also Saved Processor Status Register Used to find errors in the application program flowDefault thread runs Otherwise stated See Read Only Position Independent See also RopiSee also Rwpi See Read Write Position Independent By ARM to handle semihosting Normally set to zero on resetBlock of software code or data for an Image See Saved Processor Status Register Index Align Gbll Adrl Symbols Index-6