basic question regarding At89s51 internal data memory

Hi!

Iam new to microcontrollers...........
It is a simple question...........
but i didn't understand so ,iam asking...
please help................

In AT89s51 they will give in specifications 128 bytes of onchip ram.
but we are using 8-bit registers to access them so, we can address 256 bytes of memory.lower 128 bytes are addressed by direct and indirect addressing.,because there are memory locations which are existing there.and upper 128 bytes ae addressed by using indirect addressing.............
my doubt is: Actually those locations we are saying 0x80h-0xffh are not existing on the chip then how we can store data in them and how we can access them........

this is right or wrong i don't know.............
please clear m doubt...............

> Actually those locations we are saying 0x80h-0xffh
> are not existing on the chip then how we can store
> data in them and how we can access them........

Well, it's the same as with any non-existent memory: you simply can not store data in it... Did you really expect anything else?

JW

iam sorry...
Actually i don't know exactly what is non-existing memory.

i have seen these type of some instructions and i have asked this qtn
mov R0, #89h;
mov a,@R0;

in the above 2 statements actually we are reading data in the 0x89h location which is not existing exactly speaking..........

the upper half of internal ram actually existing but because of internal architect we can,t use of indirect addressing.

hello mkiani,

if u don't mind please explai me clearly..

I didn't understood clearly what u r telling.can u explain more please..
u r telling actually the upper 128 bytes will exist , but in the datasheet, they only told 128 bytes of internal ram....................

that example is not for a 8051 device... it`s big brother, the 8052 (and likes) actually have 256 Bytes of ram, but just the lower half (00h to 7Fh) can be accessed by direct and indirect addressing and the existing upper half, (80 to FFh) just can be accessed by indirect addressing
Direct addressing will make access to the SFRs...

remember, 8051 have 128 bytes only! and don't misunderstand what's the difference betwen direct and indirect addressing...