HT56C668 datasheet(21/104 Pages) HOLTEK

21 / 104 page

HT56R67/HT56R668/HT56C668/HT56R678/HT56C678/HT56R688

Rev. 1.20

March 4, 2010

function registers and the point where the General Pur-

pose Memory begins is reserved for future expansion

purposes, attempting to read data from these locations

will return a value of 00H.

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 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 Ad-

dressing Registers indirectly will return a result of

²00H²

and writing to the registers indirectly will result in no op-

eration.

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 address and track data. When

any operation to the relevant Indirect Addressing Regis-

ters 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.

The following example shows how to clear a section of

four RAM locations already defined as locations adres1

to adres4.

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.

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