Parameter transfer for Arm Assembly Language Calling C functions -- reprinted

Parameter transfer for Arm Assembly Language Calling C functions

Source: chinaunix blog Date: 2008.06.28 (0 comments in total)I want to comment

For the arm system, the mix CILS between functions written in different languages follows the atpcs (ARM-thumb procedure call Standard), atpcs mainly defines the parameter transfer rules for function calls and how to return the parameters from the function. For details about atpcs, see ads1.2. Online books-developer guide section 2.1. This document is about How to pass parameters when calling c Functions in assembly code, and how to correctly return parameters from C functions. Unlike x86 parameter passing rules, atpcs recommends that the function have no more than four parameters. If the number of parameters is less than or equal to 4, the parameter is composed of r0, R1, R2, r3 registers are passed. If the number of parameters is greater than 4, the parts greater than 4 must be passed through the stack. We will first discuss the situation where the number of parameters is 4. Instance 1: Test_asm_args.asm //-------------------------------------------------------------------------------- Import test_c_args; declare the test_c_args Function Area test_asm, code, readonly Export test_asm_args Test_asm_args Str lr, [Sp, #-4]! ; Save the current lR LDR r0, = 0x10; parameter 1 LDR R1, = 0x20; parameter 2 LDR R2, = 0x30; parameter 3 LDR R3, = 0x40; parameter 4 BL test_c_args; call the c Function Ldr pc, [Sp], #4; load LR into PC (return the main function) End Test_c_args.c //-------------------------------------------------------------------------------- Void test_c_args (int A, int B, int C, int D) { Printk ("test_c_args:/N "); Printk ("% 0x % 0x % 0x % 0x/N", A, B, C, D ); } Main. c //-------------------------------------------------------------------------------- Int main () { Test_asm_args (); For (;;); } The program runs from the main function. Main calls test_asm_args, test_asm_args calls test_c_args, and returns main from test_asm_args. The Code uses assembly and C to define two functions, test_asm_args and Test_c_args and test_asm_args call test_c_args. The parameter transmission method is ~ R3 writes the parameter values respectively, and then uses the BL statement. Call test_c_args. Note that the statement marked in red is used. Before test_asm_args is called, the current LR is added to the stack. After test_c_args is called, write the LR stored in the stack back to the PC so that it can be returned to the main function. What if test_c_args has eight parameters? In this case, how should we PASS Parameters in test_asm_args? Instance 2: Test_asm_args.asm //-------------------------------------------------------------------------------- Import test_c_args; declare the test_c_args Function Area test_asm, code, readonly Export test_asm_args Test_asm_args Str lr, [Sp, #-4]! ; Save the current lR LDR r0, = 0x1; parameter 1 LDR R1, = 0x2; parameter 2 LDR R2, = 0x3; parameter 3 LDR R3, = 0x4; parameter 4 LDR R4, = 0x8 STR R4, [Sp, #-4]! ; Parameter 8 into the stack LDR R4, = 0x7 STR R4, [Sp, #-4]! ; Parameter 7 LDR R4, = 0x6 STR R4, [Sp, #-4]! ; Parameter 6 LDR R4, = 0x5 STR R4, [Sp, #-4]! ; Parameter 5 into the stack BL test_c_args_lots Add SP, SP, #4; clear parameter 5 in the stack. After this statement is executed, SP points to parameter 6. Add SP, SP, #4; clear parameter 6 in the stack. After this statement is executed, SP points to parameter 7. Add SP, SP, #4; clear parameter 7 in stack. After this statement is executed, SP points to parameter 8. Add SP, SP, #4; clear parameter 8 in the stack. After this statement is executed, SP points to lR Ldr pc, [Sp], #4; load LR into PC (return the main function) End Test_c_args.c //-------------------------------------------------------------------------------- Void test_c_args (int A, int B, int C, int D, int e, int F, int g, int H) { Printk ("test_c_args_lots:/N "); Printk ("% 0x % 0x % 0x % 0x % 0x % 0x % 0x % 0x/N ", A, B, c, d, e, f, g, h ); } Main. c //-------------------------------------------------------------------------------- Int main () { Test_asm_args (); For (;;); } Most of the Code in this part is the same as the code in instance 1. The difference is the number of parameters in test_c_args and the parameter passing method in test_asm_args. In test_asm_args, parameter 1 ~ Parameter 4 still goes through R0 ~ R3 is passed, and the parameters are 5 ~ Parameter 8 is passed by pushing it into the stack. However, pay attention to the order of the four parameters in the stack, it is written to the stack in the order of parameter 8-> parameter 7-> parameter 6-> parameter 5. Until test_c_args is called, The stack content is as follows: SP-> + ---------- + | Parameter 5 | + ---------- + | Parameter 6 | + ---------- + | Parameter 7 | + ---------- + | Parameter 8 | + ---------- + | LR | + ---------- + After test_c_args returns the result, it sets sp, clears the parameters of the previous stack, loads LR into the PC, and returns the main function. After executing ldr pc, [Sp], #4 the stack content before the command is as follows: + ---------- + | Parameter 5 | + ---------- + | Parameter 6 | + ---------- + | Parameter 7 | + ---------- + | Parameter 8 | SP-> + ---------- + | LR | + ---------- + This article is from the chinaunix blog. If you want to view the original text, click:Http://blog.chinaunix.net/u2/70832/showart_1019133.html