Freescale Semiconductor, Inc.
Central Processing Unit
Indexed Addressing Modes
2.6 Indexed Addressing Modes
The CPU12 indexed modes reduce execution time and eliminate code
size penalties for using the Y index register. CPU12 indexed addressing
uses a postbyte plus zero, one, or two extension bytes after the
instruction opcode. The postbyte and extensions do the following tasks:
• Specify which index register is used.
• Determine whether a value in an accumulator is used as an offset.
• Enable automatic pre- or post-increment or decrement
• Specify use of 5-bit, 9-bit, or 16-bit signed offsets.
Table 2-2. Summary of Indexed Operations
Source
Code
Syntax
Postbyte
Code (xb)
Comments
,r
5-bit constant offset n = –16 to +15
rr can specify X, Y, SP, or PC
rr0nnnnn
111rr0zs
n,r
–n,r
Constant offset (9- or 16-bit signed)
z-0 = 9-bit with sign in LSB of postbyte(s)
1 = 16-bit
if z = s = 1, 16-bit offset indexed-indirect (see below)
rr can specify X, Y, SP, or PC
n,r
–n,r
16-bit offset indexed-indirect
rr can specify X, Y, SP, or PC
111rr011
rr1pnnnn
[n,r]
Auto pre-decrement/increment or Auto post-
decrement/increment;
p = pre-(0) or post-(1), n = –8 to –1, +1 to +8
n,–r n,+r
n,r– n,r+
rr can specify X, Y, or SP (PC not a valid choice)
Accumulator offset (unsigned 8-bit or 16-bit)
aa-00 = A
01 = B
10 = D (16-bit)
11 = see accumulator D offset indexed-indirect
rr can specify X, Y, SP, or PC
A,r
B,r
D,r
111rr1aa
111rr111
Accumulator D offset indexed-indirect
rr can specify X, Y, SP, or PC
[D,r]
68HC(9)12D60 — Rev 4.0
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Central Processing Unit
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