Texas Instruments MSC1210 - Manual (Page 205)

Incrementing and Decrementing Registers (INC, DEC)
16-11
8052 Assembly Language
16.10 Incrementing and Decrementing Registers (INC, DEC)
Two instructions, INC and DEC, can be used to increment or decrement the
value of a register, internal RAM, or SFR by 1. These instructions are rather
self-explanatory.
The INC instruction will add 1 to the current value of the specified register. If
the current value is 255, it will overflow back to 0. For example, if the accumula-
tor holds the value 240 and the INC A instruction is executed, the accumulator
is incremented to 241.
INC A ;Increment the accumulator by 1
INC R1 ;Increment R1 by 1
INC 40h ;Increment Internal RAM address 40h by 1
The DEC instruction will subtract 1 from the current value of the specified regis-
ter. If the current value is 0, it will underflow back to 255. For example, if the
accumulator holds the value 240 and the DEC A instruction is executed, the
accumulator will be decremented to 239.
DEC A ;Decrement the accumulator by 1
DEC R1 ;Decrement R1 by 1
DEC 40h ;Decrement Internal RAM address 40h by 1
Note:
Under some assembly language architectures, the INC and DEC instruc-
tions set an overflow or underflow flag when the register overflows from 255
to 0 or underflows from 0 back to 255, respectively. This is not the case with
the INC and DEC instructions in 8052 assembly language. Neither of these
instructions affects any flags whatsoever.