MC68HC16Z1CAG16 Datasheet, PDF (63/500 Pages) Freescale Semiconductor, Inc – M68HC16Z Series users manual

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4.2.1 Accumulators
The CPU16 has two 8-bit accumulators (A and B) and one 16-bit accumulator (E). In
addition, accumulators A and B can be concatenated into a second 16-bit double ac-
cumulator (D).
Accumulators A, B, and D are general-purpose registers that hold operands and re-
sults during mathematical and data manipulation operations.
Accumulator E, which can be used in the same way as accumulator D, also extends
CPU16 capabilities. It allows more data to be held within the CPU16 during operations,
simplifies 32-bit arithmetic and digital signal processing, and provides a practical 16-
bit accumulator offset indexed addressing mode.
4.2.2 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. IZ 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.
4.2.3 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 latest
stack entry. Although the stack pointer is normally incremented or decremented by
word address, it is possible to push and pull byte-sized data. Setting the stack pointer
to an odd value causes data misalignment, which reduces performance.
4.2.4 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. Address line 0 always
has a value of zero during instruction fetches to ensure that instructions are fetched
from word-aligned addresses.
M68HC16 Z SERIES
CENTRAL PROCESSOR UNIT
USER’S MANUAL
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