Inc/Dec/Inclr/Declr - Incrementation instructions

inc : Register = Register + x
dec : Register = Register - x
inclr: LastRegister = LastRegister + x
declr: LastRegister = LastRegister +x

The inc and dec instructions are used to respectively increment and decrement the value in a register by the amount of a specified immediate value. The specified value can be 4 bit, so the value added can be from 1 to 16 included.
The inclr and declr forms are compact and fast forms dedicated to increment/decrement the index register used in indirect addressing modes of the move instruction. These instructions are a kind of postincrementation/postdecrementation option. The incremented/decremented register is the last register used as an index in an indirect addressing mode. The register number is stored in the R field of the status register. The amount of the incrementation/decrementation is usualy 1 but depend on the size of the last access stored in the S field of the status register.

Forms and variants

rx = register

Instruction encoding

I = 4 bit value to add minus 1 (to add 1 : 0000, to add 16 : 1111)
D = working register number

Code Examples

Note: This example is for understanding and can be optimized.

// Example : Memory Copy 

; r0 = Memory portion to copy
; r1 = Destination buffer
; r2 = Bytes to copy

copy:  pushregs r3
loop:  move.b (r0),r3
       move.b r3,(r1)
       inc.l #1, r0
       inc.l #1, r1
       dec.l #1, r2
       bne loop
       popregs r3
       rts