hai, iam using 8051 since 2 months. iam having some doubts. 1.how much maximum external memory(ram) i can access through 8051
You need to start with the basics: read the followig documents - commonly referred to as "the bible" for the 8051: Chapter 1 - 80C51 Family Architecture: http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_ARCH_1.pdf Chapter 2 - 80C51 Family Programmer's Guide and Instruction Set: http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_PROG_GUIDE_1.pdf Chapter 3 - 80C51 Family Hardware Description: http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_HARDWARE_1.pdf
Using xdata banking you may extend the address space up to 2 MB.
"Using xdata banking you may extend the address space..." (my emphasis) For any processor, the amount of memory addressable is governed by the width of the databus (which is usually closely related to the processor's Word size). This information can be found in the Datasheet for the specific device. Note that not all 8051 derivatives today provide an external memory interface - so they're further limited to what's on the chip. You can, of course, expand the addressing range of any processor with suitable hardware and software; eg, Keil's XDATA Banking for the 8051, or the disk-based Virtual Memory found on big systems. So the question "how much maximum external memory(ram) i can access?" is really just another "how long is a piece of string?"
The simple answer is 64K for a plain 8051. More is you want to do banking. More for some variants. lees or none for other varients.