ARM程序示例.doc

EXAMPLE 1 (About LDR、LTORG) AREA Loadcon, CODE, READONLY ENTRY ; Mark first instruction to executestart BL func1 ; Branch to first subroutine BL func2 ; Branch to second subroutinestop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SWI 0x123456 ; ARM semihosting SWIfunc1 LDR r0, =42 ; = MOV R0, #42 LDR r1, =0x55555555 ; = LDR R1, PC, #offset to ; Literal Pool 1 LDR r2, =0xFFFFFFFF ; = MVN R2, #0 MOV pc, lr LTORG ; Literal Pool 1 contains ; literal Ox55555555func2 LDR r3, =0x55555555 ; = LDR R3, PC, #offset to ; Literal Pool 1 ; LDR r4, =0x66666666 ; If this is uncommented it ; fails, because Literal Pool 2 ; is out of reach MOV pc, lrLargeTable SPACE 4200 ; Starting at the current location, ; clears a 4200 byte area of memory ; to zero END ; Literal Pool 2 is emptyThe LDR pseudo-instruction generates the most efficient code for a specific constant:if the constant can be constructed with a MOV or MVN instruction, the assembler generates the appropriate instruction.if the constant cannot be constructed with a MOV or MVN instruction, the assembler:l places the value in a literal pool (a portion of memory embedded in the code to hold constant values)l generates an LDR instruction with a program-relative address that reads the constant from the literal pool.For example: LDR rn, pc, #offset to literal pool ; load register n with one word ; from the address pc + offsetYou must ensure that there is a literal pool within range of the LDR instruction generated by the assembler. Refer to Placing literal pools for more information.The offset from the pc to the constant must be: less than 4KB in ARM state, but can be in either directionforward and less than 1KB in Thumb state.When an LDR Rd,=const pseudo-instruction requires the constant to be placed in a literal pool, the assembler:l checks if the constant is available and addressable in any previous literal pools. If so, it addresses the existing constant.l attempts to place the constant in the next literal pool if it is not already available.If the next literal pool is out of range, the assembler generates an error message. In this case you must use the LTORG directive to place an additional literal pool in the code. Place the LTORG directive after the failed LDR pseudo-instruction, and within 4KB (ARM) or 1KB (Thumb). Refer to LTORG for a detailed description.EXAMPLE 2 (ADR ADRL) AREA adrlabel, CODE,READONLY ENTRY ; Mark first instruction to executeStart BL func ; Branch to subroutinestop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SWI 0x123456 ; ARM semihosting SWI LTORG ; Create a literal poolfunc ADR r0, Start ; = SUB r0, PC, #offset to Start ADR r1, DataArea ; = ADD r1, PC, #offset to DataArea ; ADR r2, DataArea+4300 ; This would fail because the offset ; cannot be expressed by operand2 ; of an ADD ADRL r2, DataArea+4300 ; = ADD r2, PC, #offset1 ; ADD r2, r2, #offset2 MOV pc, lr ; ReturnDataArea SPACE 8000 ; Starting at the current location, ; clears a 8000 byte area of memory ; to zeroENDThe assembler converts an ADR rn,label pseudo-instruction by generating:l a single ADD or SUB instruction that loads the address, if it is in rangel an error message if the address cannot be reached in a single instruction.The offset range is 255 bytes for an offset to a non word-aligned address, and 1020 bytes (255 words) for an offset to a word-aligned address. (For Thumb, the address must be word aligned, and the offset must be positive.)The assembler converts an ADRL rn,label pseudo-instruction by generating:l two data-processing instructions that load the address, if it is in rangel an error message if the address cannot be constructed in two instructions.The range of an ADRL pseudo-instruction is 64KB for a non word-aligned address and 256KB for a word-aligned address. (There is no ADRL pseudo-instruction for Thumb.)ADRL assembles to two instructions, if successful. The assembler generates two instructions even if the address could be loaded in a single instruction.EXAMPLE 3 ARM code jump table AREA Jump, CODE, READONLY ; Name this block of code CODE32 ; Following code is ARM codenum EQU 2 ; Number of entries in jump table ENTRY ; Mark first instruction to executestart ; First instruction to call MOV r0, #0 ; Set up the three parameters MOV r1, #3 MOV r2, #2 BL arithfunc ; Call the functionstop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SWI 0x123456 ; ARM semihosting SWIarithfunc ; Label the function CMP r0, #num ; Treat function code as unsigned integer MOVHS pc, lr ; If code is = num then simply return ADR r3, JumpTable ; Load address of jump table LDR pc, r3,r0,LSL#2 ; Jump to the appropriate routineJumpTable DCD DoAdd DCD DoSubDoAdd ADD r0, r1, r2 ; Operation 0 MOV pc, lr ; ReturnDoSub SUB r0, r1, r2 ; Operation 1 MOV pc, lr ; Return END ; Mark the end of this fileExample 4 String copy AREA StrCopy, CODE, READONLY ENTRY ; Mark first instruction to executestart LDR r1, =srcstr ; Pointer to first string LDR r0, =dststr ; Pointer to second string BL strcopy ; Call subroutine to do copystop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SWI 0x123456 ; ARM semihosting SWIstrcopy LDRB r2, r1,#1 ; Load byte and update address STRB r2, r0,#1 ; Store byte and update address CMP r2, #0 ; Check for zero terminator BNE strcopy ; Keep going if not MOV pc,lr ; Return AREA Strings, DATA, READWRITEsrcstr DCB First string - source,0dststr DCB Second string - destination,0 ENDExample 5 Block copy AREA Word, CODE, READONLY ; name this block of codenum EQU 20 ; set number of words to be copied ENTRY ; mark the first instruction to callstart LDR r0, =src ; r0 = pointer to source block LDR r1, =dst ; r1 = pointer to destination block MOV r2, #num ; r2 = number of words to copywordcopy LDR r3, r0, #4 ; load a word from the source and STR r3, r1, #4 ; store it to the destination SUBS r2, r2, #1 ; decrement the counter BNE wordcopy ; . copy morestop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SWI 0x123456 ; ARM semihosting SWI AREA BlockData, DATA, READWRITEsrc DCD 1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4dst DCD 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0END AREA Block, CODE, READONLY ; name this block of codenum EQU 20 ; set number of words to be copied ENTRY ; mark the first instruction to callstart LDR r0, =src ; r0 = pointer to source block LDR r1, =dst ; r1 = pointer to destination block MOV r2, #num ; r2 = number of words to copy MOV sp, #0x400 ; Set up stack pointer (r13)blockcopy MOVS r3,r2, LSR #3 ; Number of eight word multiples BEQ copywords ; Less than eight words to move? STMFD sp!, r4-r11 ; Save some working registersoctcopy LDMIA r0!, r4-r11 ; Load 8 words from the source STMIA r1!, r4-r11 ; and put them at the destination SUBS r3, r3, #1 ; Decrement the counter BNE octcopy ; . copy more LDMFD sp!, r4-r11 ; Dont need these now - restore ; originalscopywords ANDS r2, r2, #7 ; Number of odd words to copy BEQ stop ; No words left to copy?wordcopy LDR r3, r0, #4 ; Load a word from the source and STR r3, r1, #4 ; store it to the destination SUBS r2, r2, #1 ; Decrement the counter BNE wordcopy ; . copy morestop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SWI 0x123456 ; ARM semihosting SWI AREA BlockData, DATA, READWRITEsrc DCD 1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4dst DCD 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0ENDTest-and-branch macro exampleA test-and-branch operation requires two ARM instructions to implement.You can define a macro definition such as this: MACRO$label TestAndBranch $dest, $reg, $cc$label CMP $reg, #0 B$cc $dest MENDThe line after the MACRO directive is the macro prototype statement. The macro prototype statement defines the name (TestAndBranch) you use to invoke the macro. It also defines parameters ($label, $dest, $reg, and $cc). You must give values to the parameters when you invoke the macro. The assembler substitutes the values you give into the code.This macro can be invoked as follows:test TestAndBranch NonZero, r0, NE . .NonZeroAfter substitution this becomes:test CMP r0, #0 BNE NonZero . .NonZeroEXAMPLE 6 MAP & FIELDtypedef struct Point float x,y,z; Point;Point origin,oldloc,newloc;The following assembly language code is equivalent to the typedef statement above:PointBase RN r11 MAP 0,PointBasePoint_x FIELD 4Point_y FIELD 4Point_z FIELD 4The following assembly language code allocates space in memory. This is equivalent to the last line of C code:origin SPACE 12oldloc SPACE 12newloc SPACE 12You must load the base address of the data structure into the base register before you can use the labels defined in the map. For example: LDR PointBase,=origin MOV r0,#0 STR r0,Point_x MOV r0,#2 STR r0,Point_y MOV r0,#3 STR r0,Point_zis equivalent to the C code:origin.x = 0;origin.y = 2;origin.z = 3;Examples 7 ; straightforward substitution GBLS add4ff ;add4ff SETS ADD r4,r4,#0xFF ; set up add4ff $add4ff.00 ; invoke add4ff ; this produces ADD r4,r4,#0xFF00 ; elaborate substitution GBLS s1 GBLS s2 GBLS fixup GBLA count ;count SETA 14s1 SETS a$b$count ; s1 now has value a$b0000000Es2 SETS abcfixup SETS |xy$s2.z| ; fixup now has value |xyabcz|C$code| MOV r4,#16 ; but the label here is C$codeC 与汇编的混合编程The ARM C+ compilers support the asm syntax proposed in the ANSI C+ Standard, with the restriction that the string literal must be a single string. For example:asm(instruction;instruction);The inline assembler is invoked with the assembler specifier. The specifier is followed by a list of assembler instructions inside braces. For example:_asm instruction ; instruction . instructionATPCS registers, r0-r3, r12, lr, and PSR, are corrupted.(不可靠)Example 4-1 String copy#include void my_strcpy(const char *src, char *dst) int ch; _asm loop:#ifndef _thumb / ARM version LDRB ch, src, #1 STRB ch, dst, #1#else / Thumb version LDRB ch, src ADD src, #1 STRB ch, dst ADD dst, #1#endif CMP ch, #0 BNE loop int main(void) const char *a = Hello world!; char b20; my_strcpy (a, b); printf(Original string: %sn, a); printf(Copied string: %sn, b); return 0;LabelsC and C+ labels can be used in inline assembler statements. C and C+ labels can be branched to by branch instructions only in the form:Bcond labelYou cannot branch to C or C+ labels using BL.Storage declarationsAll storage can be declared in C or C+ and passed to the inline assembler using variables. Therefore, the storage declarations that are supported by armasm are not implemented.SWI and BL instructionsSWI and BL instructions must specify exactly the calling standard used. Three optional register lists follow the normal instruction fields. The register lists specify: l The registers that are the input parametersl The registers that are output parameters after returnl The registers that are corrupted by the called function.For example:SWIcond swi_num, input_regs, output_regs, corrupted_regs BLcond function, input_regs, output_regs, corrupted_regsUsageThe following points apply to using inline assembly language:稢omma is used as a separator in assembly language, so C expressions with the comma operator must be enclosed in parentheses to distinguish them:_asm ADD x, y, (f(), z)稩f you are using physical registers, you must ensure that the compiler does not corrupt them when evaluating expressions. For example:_asm MOV r0, x ADD y, r0, x / y / (x / y) overwrites r0 with the result.Because the compiler uses a function call to evaluate x / y, it:l corrupts r2, r3, ip, and lrl updates the NZCV flags in the CPSRl alters r0 and r1 with the dividend and modulo.The value in r0 is lost. You can work around this by using a C variable instead of r0: mov var,x add y, var, x / yThe compiler can detect the corruption in many cases, for example when it requires a temporary register and the register is already in use:_asm MOV ip, #3 ADDS x, x, #0x12345678 / this instruction is expanded ORR x, x, ip The compiler uses ip as a temporary register when it expands the ADD instruction, and corrupts the value 3 in ip. An error message is issued.稤o not use physical registers to address variables, even when it seems obvious that a specific variable is mapped onto a specific register. If the compiler detects this it either generates an error message or puts the variable into another register to avoid conflicts:int bad_f(int x) / x in r0 _asm ADD r0, r0, #1 / wrongly asserts that x is still in r0 return x; / x in r0This code returns x unaltered. The compiler assumes that x and r0 are two different variables, despite the fact that x is allocated to r0 on both function entry and function exit. As the assembly language code does not do anything useful, it is optimized away. The instruction should be written as: ADD x, x, #1Do not save and restore physical registers that are used by an inline assembler. The compiler will do this for you. If physical registers other than CPSR and SPSR are read without being written to, an error message is issued. For example:int f(int x) _asm STMFD sp!, r0 / save r0 - illegal: read before write ADD r0, x, 1 EOR x, r0, x LDMFD sp!, r0 / restore r0 - not needed. return x;4.2 Accessing C global variables from assembly codeGlobal variables can only b