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Preface Arm Compiler Tools Overview armclang Reference armlink Reference fromelf Reference armar Reference armasm Legacy Assembler Reference armasm Command-line Options --16 --32 --apcs=qualifier…qualifier --arm --arm_only --bi --bigend --brief_diagnostics, --no_brief_diagnostics --checkreglist --cpreproc --cpreproc_opts=option[,option,…] --cpu=list (armasm) --cpu=name (armasm) --debug --depend=dependfile --depend_format=string --diag_error=tag[,tag,…] (armasm) --diag_remark=tag[,tag,…] (armasm) --diag_style={arm|ide|gnu} (armasm) --diag_suppress=tag[,tag,…] (armasm) --diag_warning=tag[,tag,…] (armasm) --dllexport_all --dwarf2 --dwarf3 --errors=errorfile --exceptions, --no_exceptions --exceptions_unwind, --no_exceptions_unwind --execstack, --no_execstack --execute_only --fpmode=model --fpu=list (armasm) --fpu=name (armasm) -g (armasm) --help (armasm) -idir[,dir, …] --keep (armasm) --length=n --li --library_type=lib --list=file --list= --littleend -m (armasm) --maxcache=n --md --no_code_gen --no_esc --no_hide_all --no_regs --no_terse --no_warn -o filename (armasm) --pd --predefine "directive" --reduce_paths, --no_reduce_paths --regnames --report-if-not-wysiwyg --show_cmdline (armasm) --thumb --unaligned_access, --no_unaligned_access --unsafe --untyped_local_labels --version_number (armasm) --via=filename (armasm) --vsn (armasm) --width=n --xref Structure of armasm Assembly Language Modules Syntax of source lines in armasm syntax assembly l Literals ELF sections and the AREA directive An example armasm syntax assembly language module Writing A32/T32 Instructions in armasm Syntax Asse About the Unified Assembler Language Syntax differences between UAL and A64 assembly la Register usage in subroutine calls Load immediate values Load immediate values using MOV and MVN Load immediate values using MOV32 Load immediate values using LDR Rd, =const Literal pools Load addresses into registers Load addresses to a register using ADR Load addresses to a register using ADRL Load addresses to a register using LDR Rd, =label Other ways to load and store registers Load and store multiple register instructions Load and store multiple register instructions in A Stack implementation using LDM and STM Stack operations for nested subroutines Block copy with LDM and STM Memory accesses The Read-Modify-Write operation Optional hash with immediate constants Use of macros Test-and-branch macro example Unsigned integer division macro example Instruction and directive relocations Symbol versions Frame directives Exception tables and Unwind tables Using armasm armasm command-line syntax Specify command-line options with an environment v Using stdin to input source code to the assembler Built-in variables and constants Identifying versions of armasm in source code Diagnostic messages Interlocks diagnostics Automatic IT block generation in T32 code T32 branch target alignment T32 code size diagnostics A32 and T32 instruction portability diagnostics T32 instruction width diagnostics Two pass assembler diagnostics Using the C preprocessor Address alignment in A32/T32 code Address alignment in A64 code Instruction width selection in T32 code Symbols, Literals, Expressions, and Operators in a Symbol naming rules Variables Numeric constants Assembly time substitution of variables Register-relative and PC-relative expressions Labels Labels for PC-relative addresses Labels for register-relative addresses Labels for absolute addresses Numeric local labels Syntax of numeric local labels String expressions String literals Numeric expressions Syntax of numeric literals Syntax of floating-point literals Logical expressions Logical literals Unary operators Binary operators Multiplicative operators String manipulation operators Shift operators Addition, subtraction, and logical operators Relational operators Boolean operators Operator precedence Difference between operator precedence in assembly armasm Directives Reference Alphabetical list of directives armasm assembly la About armasm assembly language control directives About frame directives Directives that can be omitted in pass 2 of the as ALIAS ALIGN AREA ARM or CODE32 directive ASSERT ATTR CN CODE16 directive COMMON CP DATA DCB DCD and DCDU DCDO DCFD and DCFDU DCFS and DCFSU DCI DCQ and DCQU DCW and DCWU END ENDFUNC or ENDP ENTRY EQU EXPORT or GLOBAL EXPORTAS FIELD FRAME ADDRESS FRAME POP FRAME PUSH FRAME REGISTER FRAME RESTORE FRAME RETURN ADDRESS FRAME SAVE FRAME STATE REMEMBER FRAME STATE RESTORE FRAME UNWIND ON FRAME UNWIND OFF FUNCTION or PROC GBLA, GBLL, and GBLS GET or INCLUDE IF, ELSE, ENDIF, and ELIF IMPORT and EXTERN INCBIN INFO KEEP LCLA, LCLL, and LCLS LTORG MACRO and MEND MAP MEXIT NOFP OPT QN, DN, and SN RELOC REQUIRE REQUIRE8 and PRESERVE8 RLIST RN ROUT SETA, SETL, and SETS SPACE or FILL THUMB directive TTL and SUBT WHILE and WEND WN and XN armasm-Specific A32 and T32 Instruction Set Featur armasm support for the CSDB instruction A32 and T32 pseudo-instruction summary ADRL pseudo-instruction CPY pseudo-instruction LDR pseudo-instruction MOV32 pseudo-instruction NEG pseudo-instruction UND pseudo-instruction Appendixes

Stack implementation using LDM and STM

F3.16 Stack implementation using LDM and STM

You can use the LDM and STM instructions to implement pop and push operations respectively. You use a suffix to indicate the stack type.

The load and store multiple instructions can update the base register. For stack operations, the base register is usually the stack pointer, SP. This means that you can use these instructions to implement push and pop operations for any number of registers in a single instruction.

The load and store multiple instructions can be used with several types of stack:

Descending or ascending

The stack grows downwards, starting with a high address and progressing to a lower one (a descending stack), or upwards, starting from a low address and progressing to a higher address (an ascending stack).

Full or empty

The stack pointer can either point to the last item in the stack (a full stack), or the next free space on the stack (an empty stack).

To make it easier for the programmer, stack-oriented suffixes can be used instead of the increment or decrement, and before or after suffixes. The following table shows the stack-oriented suffixes and their equivalent addressing mode suffixes for load and store instructions:

Table F3-6 Stack-oriented suffixes and equivalent addressing mode suffixes

Stack-oriented suffix For store or push instructions For load or pop instructions
FD (Full Descending stack) DB (Decrement Before) IA (Increment After)
FA (Full Ascending stack) IB (Increment Before) DA (Decrement After)
ED (Empty Descending stack) DA (Decrement After) IB (Increment Before)
EA (Empty Ascending stack) IA (Increment After) DB (Decrement Before)

The following table shows the load and store multiple instructions with the stack-oriented suffixes for the various stack types:

Table F3-7 Suffixes for load and store multiple instructions

Stack type Store Load
Full descending STMFD (STMDB, Decrement Before) LDMFD (LDM, increment after)
Full ascending STMFA (STMIB, Increment Before) LDMFA (LDMDA, Decrement After)
Empty descending STMED (STMDA, Decrement After) LDMED (LDMIB, Increment Before)
Empty ascending STMEA (STM, increment after) LDMEA (LDMDB, Decrement Before)

For example:

    STMFD    sp!, {r0-r5}  ; Push onto a Full Descending Stack
    LDMFD    sp!, {r0-r5}  ; Pop from a Full Descending Stack

Note:

The Procedure Call Standard for the Arm® Architecture (AAPCS), and armclang always use a full descending stack.

The PUSH and POP instructions assume a full descending stack. They are the preferred synonyms for STMDB and LDM with writeback.

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