Rev. 1.10
30
November 26, 2019
Rev. 1.10
31
November 26, 2019
HT68FB240
USB Low Speed Flash MCU
Special Function Register Description
Most of the Special Function Register details will be described in the relevant functional sections;
however several registers require a separate description in this section.
Indirect Addressing Register – 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 from Bank 0 while the IAR1 and MP1 register pair can
access data from any 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
Two 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 and IAR1 are used to
access data from all banks according to BP register. Direct Addressing can only be used with Bank 0,
all other Banks must be addressed indirectly using MP1 and IAR1.
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
Data Memory addresses.
