ARM GAS fac.s page 1 1 // 2 // Factorial (recursive) function 3 // 4 5 /* -------------------------------------------------- 6 Define required labels for EGTAPI 7 -------------------------------------------------- */ 8 .global main, Stop, CodeEnd, DataStart, DataEnd 9 .global fac, k, result 10 11 /* -------------------------------------------------- 12 Begin of the program instructions 13 -------------------------------------------------- */ 14 .text 15 main: 16 17 // result = fac(k) 18 19 /* ------------------------------------------------- 20 Pass parameter k (using stack) 21 ------------------------------------------------- */ 22 0000 000000E3 movw r0, #:lower16:k 23 0004 000040E3 movt r0, #:upper16:k 24 0008 000090E5 ldr r0, [r0] 25 000c 04002DE5 push {r0} 26 27 /* ---------------------------------------------------------------- 28 call result = fac(k) 29 ---------------------------------------------------------------- */ 30 0010 FEFFFFEB bl fac 31 0014 04D08DE2 add sp, sp, #4 // Clean up the parameter k 32 33 /* ----------------------------------------------------------------- 34 Save return value (in r0) to variable result 35 ----------------------------------------------------------------- */ 36 0018 001000E3 movw r1, #:lower16:result // Do NOT use r0 !!! 37 001c 001040E3 movt r1, #:upper16:result // (Because r0 contains the return value) 38 39 0020 000081E5 str r0, [r1] // This will store return value in sum 40 41 Stop: 42 0024 0000A0E1 nop // Stop point of main( ) 43 44 45 46 47 48 /* ---------------------------------------------------------------- 49 Function int fac(int n) 50 51 Stack frame structure: 52 53 old FP <--------------------------------- FP 54 old LR (ret addr) 55 n +8 56 57 Body of fac(n) ARM GAS fac.s page 2 58 59 if ( n == 0 ) 60 return 1; 61 else 62 return n*fac(n-1); 63 ---------------------------------------------------------------- */ 64 65 fac: 66 67 // When fac begins, we will have: n on the stack 68 69 70 /* ========================================================== 71 Function Prelude: complete the stack frame structure 72 ========================================================== */ 73 0028 04E02DE5 push {lr} // Save LR (return address) 74 002c 04B02DE5 push {fp} // Save FP (used by caller) 75 0030 0DB0A0E1 mov fp, sp // Mark the stack top location before 76 // allocating any local variables 77 0034 00D04DE2 sub sp, sp, #0 // Allocate 0 int variables on the stack 78 // (I could delete this instruction....) 79 80 81 82 /* =============================================== 83 We completed the stack frame 84 Now we can write the function body 85 =============================================== */ 86 // if ( n == 0 ) 87 0038 08009BE5 ldr r0, [fp, #8] // r0 = n 88 89 003c 000050E3 cmp r0, #0 // Check n == 0 90 0040 0300001A bne else // n != 0 --> Goto "else" part 91 92 // return 1 93 94 ///// put return value 1 in return location r0 95 0044 0100A0E3 mov r0, #1 96 97 /* ============================================================= 98 Function Postlude: de-allocate local variable and restore FP 99 ============================================================= */ 100 0048 0BD0A0E1 mov sp, fp // De-allocate local variables 101 004c 04B09DE4 pop {fp} // Restore fp 102 0050 04F09DE4 pop {pc} // Return to the caller 103 104 else: 105 // Compute: n*fac(n-1) 106 107 //// Compute fac(n-1) first 108 0054 08009BE5 ldr r0, [fp, #8] // r0 = n 109 0058 010040E2 sub r0, r0, #1 // r0 = n-1 110 111 005c 04002DE5 push {r0} // pass (n-1) to fac on stack 112 0060 FEFFFFEB bl fac 113 0064 04D08DE2 add sp, sp, #4 // Clean up parameter (n-1) from stack 114 ARM GAS fac.s page 3 115 //// ** Right now, r0 = fac(n-1) !!! 116 117 0068 08109BE5 ldr r1, [fp, #8] // r0 = n 118 006c 910000E0 mul r0, r1, r0 // r0 = n*fac(n-1) 119 // NOTE: r0 has the correct return value !!! 120 121 // return n*fac(n-1) in r0 122 /* ============================================================= 123 Function Postlude: de-allocate local variable and restore FP 124 ============================================================= */ 125 0070 0BD0A0E1 mov sp, fp // De-allocate local variables 126 0074 04B09DE4 pop {fp} // Restore fp 127 0078 04F09DE4 pop {pc} // Return to the caller 128 129 130 131 132 CodeEnd: 133 007c 0000A0E1 nop 134 135 /* -------------------------------------------------- 136 Begin of the permanent program variables 137 -------------------------------------------------- */ 138 .data 139 DataStart: 140 141 0000 03000000 k: .4byte 3 142 0004 00000000 result: .skip 4 143 144 DataEnd: 145 146 .end ARM GAS fac.s page 4 DEFINED SYMBOLS fac.s:15 .text:0000000000000000 main fac.s:41 .text:0000000000000024 Stop fac.s:132 .text:000000000000007c CodeEnd fac.s:139 .data:0000000000000000 DataStart fac.s:144 .data:0000000000000008 DataEnd fac.s:65 .text:0000000000000028 fac fac.s:141 .data:0000000000000000 k fac.s:142 .data:0000000000000004 result fac.s:22 .text:0000000000000000 $a fac.s:104 .text:0000000000000054 else fac.s:142 .data:0000000000000004 $d NO UNDEFINED SYMBOLS