HT46R12A
operation. These areas may function as normal pro-
gram memory depending upon requirements.
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1
Stack Register - STACK
P
1
This is a special part of the memory which is used to
save the contents of the program counter only. The
stack is organized into 8 levels and is neither part of the
data nor part of the program space, and is neither read-
able nor writeable. The activated level is indexed by the
stack pointer, SP, and is neither readable nor writeable.
At a subroutine call or interrupt acknowledge signal, the
contents of the program counter are pushed onto the
stack. At the end of a subroutine or an interrupt routine,
indicated by a return instruction, RET or RETI, the pro-
gram counter is restored to its previous value from the
stack. After a chip reset, the SP will point to the top of the
stack.
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C
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C
L
T
B
L
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T
B
L
H
0
0
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B
H
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A
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U
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I
N
T
C
0
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0
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D
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E
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1
1
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1
1
1
1
0
1
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T
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1
T
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1
C
P
A
If the stack is full and a non-masked interrupt takes
place, the interrupt request flag will be recorded but the
acknowledge signal will be inhibited. When the stack
pointer is decremented, using RET or RETI, the inter-
rupt will be serviced. This feature prevents a stack over-
flow allowing the programmer to use the structure more
easily. In a similar case, if the stack is full and a ²CALL²
is subsequently executed, a stack overflow will occur
and the first entry will be lost as only the most recent 8
return addresses are stored.
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y
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1
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B
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1
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D
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0
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1
C
Data Memory - RAM
1
E
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C
1
1
F
H
The data memory has a capacity of 115´8 bits, and is di-
vided into two functional groups, namely the special
function registers and the general purpose data mem-
ory (88´8 bits), most of which are readable/writeable,
although some are read only.
2
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8
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0
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D
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The unused space before address 28H is reserved for
future expansion usage and reading these locations will
obtain a result of ²00H². The general purpose data
memory, addressed from 28H to 7FH is used for data
and control information under instruction commands.
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:
U
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(
8
8
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)
All of the data memory areas can handle arithmetic,
logic, increment, decrement and rotate operations di-
rectly. Except for some dedicated bits, each bit in the
data memory can be set and reset by ²SET [m].i² and
²CLR [m].i². They are also indirectly accessible through
the memory pointer registers, MP0 and MP1.
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7
F
H
RAM Mapping
Rev. 1.00
8
August 3, 2007
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
