Next, we arrive at mask-field.  This accessor returns a number which has the masked-off bits set to zero.  It is also setf-able, meaning that it can be used to emulate the behaviour of deposit-field, but changing the integer in place.  Note that in the setf case, as with deposit-field, the byte specifier acts on the inserted number as well.  I’m going to show the Lisp code and the equivalent C code, for clarity:
 

CL-USER> (let* ((orig #xffffffff)
                (mask-width 10)
                (mask-shift 5)
                (mask (byte mask-width mask-shift)))
           (format t "Masked off part is:  #x~X~%"
                   (mask-field mask orig))
           (format t "orig is unchanged:   #x~X~%"
                   orig)
           (format t "deposit-field #x100: #x~X~%"
                   (deposit-field #x100 mask orig))
           (format t "Setting the masked part to #x100...~%")
           (setf (mask-field mask orig) #x100)
           (format t "orig is now:         #x~X~%" 
                   orig))
Masked off part is:  #x7FE0
orig is unchanged:   #xFFFFFFFF
deposit-field #x100: #xFFFF811F
Setting the masked part to #x100...
orig is now:         #xFFFF811F
NIL

And the C:
 

#include <stdio.h>
#include <stdint.h>

int main(void)
{

    uint32_t orig = 0xffffffff;
    uint32_t mask = 0x7fe0;
    uint32_t inserting = 0x100;

    printf("Masked off part is:   0x%x\n", orig & mask);
    printf("orig is unchanged:    0x%x\n", orig);

    printf("Setting the masked part to 0x100...\n");
    orig = ( orig & ~mask ) | (inserting & mask);
    printf("orig is now           0x%x\n", orig);
    return 0;
}

producing output:
 

$ ./mask-field 
Masked off part is:   0x7fe0
orig is unchanged:    0xffffffff
Setting the masked part to 0x100...
orig is now           0xffff811f