clz.asm

uCOSII 在LPC3180上的移植代码

; File : clz.asm
; By   : Eric Shufro
; For  : ARM926EJ-S

; Note : This routine will also work on any other ARM CPU
;        with instruction set version >= 5

;********************************************************************************************************
;                                      CODE GENERATION DIRECTIVES
;********************************************************************************************************

    RSEG CODE:CODE:NOROOT(2)
    CODE32

;********************************************************************************************************
;                                      PROTOTYPES (C equivalent in ASM)
;********************************************************************************************************

    PUBLIC  CountLeadingZeros       ;declare the below sub routine public
                                    ;so that it can be seen by the C compiler.
                                    ;(Similar to prototyping C functions in .h files)

;********************************************************************************************************
;                    Count Leading Zero's
;
; Description  : This routine is called from OS_CPU_IRQ_ISR_Handler and
;                OS_CPU_FIQ_ISR_Handler (bsp.c) in order to quickly
;                determine the bit position of most significant '1' in
;                a given 32 bit value
;
; Arguments    : A 32 bit value ('data') that is to have its leading zero's
;                counted (High order to Low order). This value arrives in
;                register R0
;
; Returns      : A 32 bit value ('ret_val') representing the number of zeros
;                found present before reaching the most significant set bit in
;                'data' (Counting from high order to low order). This result
;                is stored in register R1, but copied to, and returned from R0.
;
;                   Ex: If Bit 5 is set, then 'ret_val' would equal 0x1A,
;                       meaning there are 26 leading zero's before bit 5.
;                       OR (31 minus bit 5 = 26). Note: Zero set bits will
;                       return 32. So the above 31-bit formula doesnt work
;                       for the case when no bits are set.
;
; Notes        : This sub routine should be prototyped in a .c or .h file
;                as follows:
;
;                extern  INT32U  CountLeadingZeros(INT32U val);
;
; Example      : From C code, this routine may be called as follows:
;
;                INT32U  leading_zeros;
;
;
;                leading_zeros = CountLeadingZeros(val);
;********************************************************************************************************


CountLeadingZeros:
    CLZ R1,R0                       ;Perform the count leading zero's operation
    MOV R0, R1                      ;Copy the return data to R0.
    BX  LR                          ;Return to the calling function

    END