
CPU16
REFERENCE MANUAL
SYSTEM RESOURCES
MOTOROLA
3-3
The CPU16 accumulators can perform the same operations as M68HC11 accumula-
tors of the same names, but the CPU16 instruction set provides additional 8-bit, 16-
bit, and 32-bit accumulator operations. See
SECTION 5 INSTRUCTION SET
for more
information.
3.2.2 Index Registers
The CPU16 has three 16-bit index registers (IX, IY, and IZ). Each index register has
an associated 4-bit extension field (XK, YK, and ZK).
Concatenated registers and extension fields provide 20-bit indexed addressing and
support data structure functions anywhere in the CPU16 address space.
IX and IY can perform the same operations as M68HC11 registers of the same names,
but the CPU16 instruction set provides additional indexed operations.
IZ can perform the same operations as IX and IY, and also provides an additional in-
dexed addressing capability that replaces M68HC11 direct addressing mode. Initial IZ
and ZK extension field values are included in the RESET exception vector, so that ZK
: IZ can be used as a direct page pointer out of reset. See
SECTION 4 DATA TYPES
AND ADDRESSING MODES
and
SECTION 9 EXCEPTION PROCESSING
for more
information.
3.2.3 Stack Pointer
The CPU16 stack pointer (SP) is 16 bits wide. An associated 4-bit extension field (SK)
provides 20-bit stack addressing.
Stack implementation in the CPU16 is from high to low memory. The stack grows
downward as it is filled. SK : SP are decremented each time data is pushed on the
stack, and incremented each time data is pulled from the stack.
SK : SP point to the next available stack address, rather than to the address of the lat-
est stack entry. Although the stack pointer is normally incremented or decremented by
word address, it is possible to push and pull byte-sized data; however, setting the
stack pointer to an odd value causes misalignment, which affects performance. See
SECTION 4 DATA TYPES AND ADDRESSING MODES
and
SECTION 5 INSTRUC-
TION SET
for more information.
3.2.4 Program Counter
The CPU16 program counter (PC) is 16 bits wide. An associated 4-bit extension field
(PK) provides 20-bit program addressing.
CPU16 instructions are fetched from even word boundaries. Bit 0 of the PC always has
a value of zero, to assure that instruction fetches are made from word-aligned ad-
dresses. See
SECTION 7 INSTRUCTION PROCESS
for more information.
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