Clear_x_d ($F548)
This routine clears the block of memory starting at the contained in the X register to zero.
ENTRY:
X-reg points to the start of the RAM to be cleared. D-reg = number of bytes to clear minus 1
X-reg points to the start of the RAM to be cleared.
D-reg = number of bytes to clear minus 1
EXIT:
D-reg = $FFFF
Source