HT46R005 datasheet(17/62 Pages) HOLTEK

17 / 62 page

HT48R005/HT46R005

Rev. 1.00

November 29, 2011

Special Function Registers

To ensure successful operation of the microcontroller,

certain internal registers are implemented in the Data

Memory area. These registers ensure correct operation

of internal functions such as timers, interrupts, etc., as

well as external functions such as I/O data control. The

location of these registers within the Data Memory be-

gins at the address

²00H². Any unused Data Memory lo-

cations between these special function registers and the

point where the General Purpose Memory begins is re-

served and attempting to read data from these locations

will return a value of

²00H².

Indirect Addressing Registers

- IAR0, IAR1

The Indirect Addressing Registers, IAR0 and IAR1, al-

though having their locations in normal RAM register

space, do not actually physically exist as normal regis-

ters. The method of indirect addressing for RAM data

manipulation uses these Indirect Addressing Registers

and Memory Pointers, in contrast to direct memory ad-

dressing, where the actual memory address is speci-

fied. 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 corre-

sponding Memory Pointer, MP0 or MP1. Acting as a

pair, IAR0 with MP0 and IAR1 with MP1 can together ac-

cess data from the Data Memory. As the Indirect Ad-

dressing Registers are not physically implemented,

reading the Indirect Addressing Registers indirectly will

return a result of

²00H² and writing to the registers indi-

rectly will result in no operation.

Memory Pointers

- MP0, MP1

Two Memory Pointers, known as MP0 and MP1 are pro-

vided. These Memory Pointers are physically imple-

mented in the Data Memory and can be manipulated in

the same way as normal registers providing a conve-

nient way with which to indirectly address and track

data. When any operation to the relevant Indirect Ad-

dressing Registers is carried out, the actual address that

the microcontroller is directed to, is the address speci-

fied by the related Memory Pointer. The following exam-

ple shows how to clear a section of four Data Memory

locations already defined as locations adres1 to adres4.

Accumulator

- ACC

The Accumulator is central to the operation of any

microcontroller and is closely related with operations

carried out by the ALU. The Accumulator is the place

where all intermediate results from the ALU are stored.

Without the Accumulator it would be necessary to write

the result of each calculation or logical operation such

as addition, subtraction, shift, etc., to the Data Memory

resulting in higher programming and timing overheads.

Data transfer operations usually involve the temporary

storage function of the Accumulator; for example, when

transferring data between one user defined register and

another, it is necessary to do this by passing the data

through the Accumulator as no direct transfer between

two registers is permitted.

· 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.

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