PendSV中断里有一句不懂，谁来解释一下

__asm void xPortPendSVHandler( void )
{
extern uxCri ticalNesting;
extern pxCurrentTCB;
extern vTaskSwitchContext;
PRESERVE8
mrs r0, psp
i***
/* Get the location of the current TCB. */
ldr r3, =pxCurrentTCB
ldr r2, [r3]
/* Is the task using the FPU context? If so, push high vfp registers. */
tst r14, #0x10
it eq
vstmdbeq r0!, {s16-s31}
/* Save the core registers. */
stmdb r0!, {r4-r11, r14}
/* Save the new top of stack into the first member of the TCB. */
str r0, [r2]
stmdb sp!, {r3}
mov r0, #configMAX_SYSCALL_INTERRUPT_PRIORITY
msr basepri, r0
d***
i***
bl vTaskSwitchContext
mov r0, #0
msr basepri, r0
ldmia sp!, {r3}
/* The first item in pxCurrentTCB is the task top of stack. */
ldr r1, [r3]
ldr r0, [r1]
/* Pop the core registers. */
ldmia r0!, {r4-r11, r14}
/* Is the task using the FPU context? If so, pop the high vfp registers
too. */
tst r14, #0x10
it eq
vldmiaeq r0!, {s16-s31}
msr psp, r0
i***
#ifdef WORKAROUND_PMU_CM001 /* XMC4000 specific errata */
#if WORKAROUND_PMU_CM001 == 1
push { r14 }
pop { pc }
nop
#endif
#endif
bx r14
}
stmdb sp!, {r3} //请问这句保存r3为什么要用sp？而不是psp?