EXC_RETURN_CMD_PSP EQU 0xFFFFFFFD ; EXC_RETURN using PSP for ARM Cortex. If Link register contains this value it indicates the PSP was used before the SVC, otherwise the MSP was used.
IMPORT C_SVC_Handler
LDR R0, =EXC_RETURN_CMD_PSP ; Load the EXC_RETURN into R0 to be able to compare against LR to determine stack pointer used.
CMP R0, LR ; Compare the link register with R0. If equal then PSP was used, otherwise MSP was used before SVC.
BNE UseMSP ; Branch to code fetching SVC arguments using MSP.
MRS R1, PSP ; Move PSP into R1.
B Call_C_SVC_Handler ; Branch to Call_C_SVC_Handler below.
UseMSP
MRS R1, MSP ; MSP was used, therefore Move MSP into R1.
Call_C_SVC_Handler
LDR R0, [R1, #24] ; The arguments for the SVC was stacked. R1 contains Stack Pointer, the values stacked before SVC are R0, R1, R2, R3, R12, LR, PC (Return address), xPSR.
; R1 contains current SP so the PC of the stacked frame is at SP + 6 words (24 bytes). We load the PC into R0.
SUBS R0, #2 ; The PC before the SVC is in R0. We subtract 2 to get the address prior to the instruction executed where the SVC number is located.
LDRB R0, [R0] ; SVC instruction low octet: Load the byte at the address before the PC to fetch the SVC number.
LDR R2, =C_SVC_Handler ; Load address of C implementation of SVC handler.
BX R2 ; Branch to C implementation of SVC handler. R0 is now the SVC number, R1 is the StackPointer where the arguments (R0-R3) of the original SVC are located.
ALIGN
}
#elif defined ( __GNUC__ )
void __attribute__ (( naked )) SVC_Handler(void)
{
const uint32_t exc_return = 0xFFFFFFFD; // EXC_RETURN using PSP for ARM Cortex. If Link register contains this value it indicates the PSP was used before the SVC, otherwise the MSP was used.
__asm volatile(
"cmp lr, %0\t\n" // Compare the link register with argument 0 (%0), which is exc_return. If equal then PSP was used, otherwise MSP was used before SVC.
"bne UseMSP\t\n" // Branch to code fetching SVC arguments using MSP.
"mrs r1, psp\t\n" // Move PSP into R1.
"b Call_C_SVC_Handler\t\n" // Branch to Call_C_SVC_Handler below.
"UseMSP: \t\n" //
"mrs r1, msp\t\n" // MSP was used, therefore Move MSP into R1.
"Call_C_SVC_Handler: \t\n" //
"ldr r0, [r1, #24]\t\n" // The arguments for the SVC was stacked. R1 contains Stack Pointer, the values stacked before SVC are R0, R1, R2, R3, R12, LR, PC (Return address), xPSR.
// R1 contains current SP so the PC of the stacked frame is at SP + 6 words (24 bytes). We load the PC into R0.
"sub r0, r0, #2\t\n" // The PC before the SVC is in R0. We subtract 2 to get the address prior to the instruction executed where the SVC number is located.
"ldrb r0, [r0]\t\n" // SVC instruction low octet: Load the byte at the address before the PC to fetch the SVC number.
"bx %1\t\n" // Branch to C implementation of SVC handler, argument 1 (%1). R0 is now the SVC number, R1 is the StackPointer where the arguments (R0-R3) of the original SVC are located.
".align\t\n"
:: "r" (exc_return), "r" (C_SVC_Handler) // Argument list for the gcc assembly. exc_return is %0, C_SVC_Handler is %1.
: "r0", "r1" // List of register maintained manually.
);
}
#else
#error Compiler not supported.
#endif
__asm void SVCHandler(void)
{
IMPORT SVCHandler_main
TST lr, #4
ITE EQ
MRSEQ R0, MSP
MRSNE R0, PSP
B SVCHandler_main
}
void SVCHandler_main(unsigned int * svc_args)
{
unsigned int svc_number;
/*
* Stack contains:
* R0, R1, R2, R3, R12, R14, the return address and xPSR
* First argument (R0) is svc_args[0]
*/
svc_number = ((char *)svc_args[6])[-2];
switch(svc_number)
{
case SVC_00:
/* Handle SVC 00 */
break;
case SVC_01:
/* Handle SVC 01 */
break;
default:
/* Unknown SVC */
break;
}
}
#define SVC_00 0x00
#define SVC_01 0x01
void __svc(SVC_00) svc_zero(const char *string);
void __svc(SVC_01) svc_one(const char *string);
int call_system_func(void)
{
svc_zero("String to pass to SVC handler zero");
svc_one("String to pass to a different OS function");
}
在 ARM 狀態(tài)時(shí)為 0。
ARM 和 Thumb 指令集均有 SVC 指令。 在 Thumb 狀態(tài)下調(diào)用 SVC 時(shí)，必須考慮以下情況：
指令的地址在 lr–2，而不在 lr–4。
該指令本身為 16 位，因而需要半字加載，請(qǐng)參閱Figure 6.3。
在 ARM 狀態(tài)下，SVC 編號(hào)以 8 位存儲(chǔ)，而不是 24 位。
Example 6.8. SVC 處理程序
PRESERVE8
AREA SVC_Area, CODE, READONLY
EXPORT SVC_Handler IMPORT C_SVC_Handler
T_bit EQU 0x20 ; Thumb bit (5) of CPSR/SPSR.
SVC_Handler
STMFD sp!, {r0-r3, r12, lr} ; Store registers
MOV r1, sp ; Set pointer to parameters
MRS r0, spsr ; Get spsr
STMFD sp!, {r0, r3} ; Store spsr onto stack and another
; register to maintain 8-byte-aligned stack
TST r0, #T_bit ; Occurred in Thumb state?
LDRNEH r0, [lr,#-2] ; Yes: Load halfword and...
BICNE r0, r0, #0xFF00 ; ...extract comment field
LDREQ r0, [lr,#-4] ; No: Load word and...
BICEQ r0, r0, #0xFF000000 ; ...extract comment field
; r0 now contains SVC number
; r1 now contains pointer to stacked registers
BL C_SVC_Handler ; Call main part of handler
LDMFD sp!, {r0, r3} ; Get spsr from stack
MSR SPSR_cxsf, r0 ; Restore spsr
LDMFD sp!, {r0-r3, r12, pc}^ ; Restore registers and return
ARM SVC.zip
(3.58 KB, 下載次數(shù): 9)