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Microcontroller-Based Digital System Design
Chapter 3 - Page 46
There are two basic variants of PSH and PUL: one for byte-registers (A, B,
CCR) and another for word-registers (D, X, Y). Note that neither SP nor
PC can be pushed or pulled. Note also that the same stack convention
used by our simple computer is used here: the stack pointer (SP) points to
the location in which the top stack item is stored (for word-length items,
SP points to the high byte of the top stack item). Generally, PSH and PUL
do not affect any of the condition code bits – with the obvious exception of
PULC, which affects all the condition code bits.
Table 3-9 Data Transfer Group: Stack Manipulation Instructions.
Description
Mnemonic
Push
PSHrb
register
rb = A, B, C
onto stack
PSHrw
rw = D, X, Y
Pull (pop)
register
from stack
PULrb
rb = A, B, C
Operation
(SP) ← (SP) – 1
((SP)) ← (rb)
(SP) ← (SP) – 1
((SP)) ← (rwl)
(SP) ← (SP) – 1
((SP)) ← (rwh)
(rb) ← ((SP))
(SP) ← (SP) + 1
CC
–
–
*
Mode
~
PSHA
Examples
ž
2
PSHB
ž
2
PSHC
ž
2
PSHD
ž
2
PSHX
ž
2
PSHY
ž
2
PULA
ž
3
PULB
ž
3
PULC
ž
3
ž
3
ž
3
ž
3
PULD
(rwh) ← ((SP))
–
(SP) ← (SP) + 1
PULX
(rwl) ← ((SP))
PULY
(SP) ← (SP) + 1
* PULC affects all the condition code bits, with the exception of X, which cannot
be set by a software instruction once it is cleared.
PULrw
rw = D, X, Y
3.8.2
Arithmetic Group Instructions
Instructions that perform an arithmetic operation (add, subtract, multiply,
divide) are broadly classified here as belonging to the arithmetic group.
As one might guess, most of these instructions affect all of the condition
code bits (with a few notable exceptions).
Table 3-10 lists the variations of add (ADD) and subtract (SUB) of which
the 68HC12 is capable. The “with carry” versions (ADC and SBC) are
provided for implementing extended (or “infinite”) precision add or subtract
routines; in Chapter 4, we will learn how to write such routines. For the
ADC instruction, the “C” bit of the condition code register is interpreted as
a carry propagated forward, and is therefore added to the result. For the
SBC instruction, the “C” bit is interpreted as a borrow propagated forward,
and is therefore subtracted from the result. The “astute digijock(ette)” will
Preliminary Draft
add
ADD
add with carry
ADC
subtract
SUB
subtract with carry
SBC
©2001 by D. G. Meyer