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UM0404 The central processing unit (CPU)
3.4.17 The constant ones register ONES
All bits of this bit-addressable register are fixed to '1' by hardware. This register is read only.
Register ONES can be used as a register-addressable constant of all ones, for bit
manipulation or mask generation. It can be accessed via any instruction which is capable of
addressing an SFR.
ONES (FF1Eh / 8Fh) SFR Reset Value:FFFFh
Example
Mask for FFFFh values used to increment or decrement memory:
1514131211109876543210
1111111111111111
RRRRRRRRRRRRRRRR
sub mem, ones ;mem=mem+1
;increments the memory location in one instruction
;instead of three, as described below
mov R13, mem ;mem -> R13
add R13, #1 ;R13 + 1
mov mem, R13 ;R13 -> mem
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