hi Guru
Letz start from the very basic of memory accesses. When u want to access a single memory location at a time, what u do is tht u just specify the memory location (and the data to be written in case itz a write operation) and that it will be done. Again if u want to access another location u do the same. U know when u give address this way (one at a time), then each time the access will take same time (including accessing that location + reading/writing).
But since we know memory accesses are very slow compared to CPU speed, so a lot of time is wasted in case u want to read say contiguous memory locations and u provide those contiguous locations one at a time.
We can see it this way....say a memory read access takes say N memory cycles ...then if u want to read say M locations n u provide one address at a time...then it will take N*M memory cycles (ideally) for the entire access. Right. But if u really c its a whole lot of time....n we r not gaining anything even if the locations being contiguous.
By this wat i mean to say is that normally in SDRAMs, the access to a memory location is by first opening a particular row, followed by accessing the intended column in that row...n then that row is closed after access. So if u want to read say 4 contiguous locations a time (meaning 4 adjacent columns say N to N+4 of the same row say row no. R) then if u provide all those locations one at a time, then for every access, it will first take some time to open row R , then will access the column N, then will take time to close row R. Again for accessing column N+1 , it will again take time to open row R, then some time to access column N+1, then again will close row R.....
SO u can see unnecessarily its opening n closing the same row again n again n wasting time in that ( when we know that the locations are adjacent )....So a better option is to go for so called BURST MODE...where u just need to give input for once only in the form ofrow address, 1st column address, n the no. of columns to access. Wat SDRAM will do is tht it will open the row for the first time, n then will access columns one after the other without closing the row....n when finally all columns r accessed it will close that row.....
Here watever i said may be not exactly correct in terms of say u cant access any no. of colums (normally its 2 or 4 or 8 or the entire row)....but overall i suppose u'll wat i mean to say....
in case any doubts ...feel free to ask.....
u can also go thru this pdf...plz go thru the timing diagrams for Burst mode access (read/write)
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