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HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
Special Function Register Description
Most of the Special Function Register details will be described in the relevant functional section.
However, several registers require a separate description in this section.
Indirect Addressing Registers – IAR0, IAR1
The Indirect Addressing Registers, IAR0 and IAR1, although having their locations in normal RAM
register space, do not actually physically exist as normal registers. The method of indirect addressing
for RAM data manipulation uses these Indirect Addressing Registers and Memory Pointers, in
contrast to direct memory addressing, where the actual memory address is specified. Actions on the
IAR0 and IAR1 registers will result in no actual read or write operation to these registers but rather
to the memory location specified by their corresponding Memory Pointers, MP0 or MP1. Acting as
a pair, IAR0 and MP0 can together access data only from Bank 0 while the IAR1 register together
with MP1 register pair can access data from any Data Memory bank. As the Indirect Addressing
Registers are not physically implemented, reading the Indirect Addressing Registers indirectly will
return a result of “00H” and writing to the registers indirectly will result in no operation.
Memory Pointers – MP0, MP1
The Memory Pointers, known as MP0 and MP1, are provided. These Memory Pointers are
physically implemented in the Data Memory and can be manipulated in the same way as normal
registers providing a convenient way with which to address and track data. When any operation to
the relevant Indirect Addressing Registers is carried out, the actual address that the microcontroller
is directed to is the address specified by the related Memory Pointer. MP0, together with Indirect
Addressing Register, IAR0, are used to access data from Bank 0, while MP1 together with IAR1 are
used to access data from all data banks according to the corresponding BP register.
The following example shows how to clear a section of four Data Memory locations already defined
as locations adres1 to adres4.
Indirect Addressing Program Example
data .section ‘data’
adres1 db ?
adres2 db ?
adres3 db ?
adres4 db ?
block db ?
code .section at 0 ‘code’
org 00h
start:
mov a,04h ; setup size of block
mov block,a
mov a,offset adres1 ; Accumulator loaded with first RAM address
mov mp0,a ; setup memory pointer with first RAM address
loop:
clr IAR0 ; clear the data at address defined by MP0
inc
mp0
;
increment
memory
pointer
sdz block ; check if last memory location has been cleared
jmp loop
continue:
:
The important point to note here is that in the example shown above, no reference is made to specific
RAM addresses.
