Direct Addressing

Direct addressing is so-named because the value to be stored in memory is obtained by directly retrieving it from another memory location. 

For example :-      MOV A, #30h

 This instruction will read the data out of Internal RAM address 30 (hexidecimal) and store it in the Accumulator. Direct addressing is generally fast since, although the value to be loaded isn't included in the instruction, it is quickly accessable since it is stored in the 8051s Internal RAM. It is also much more flexible than Immediate Addressing since the value to be loaded is whatever is found at the given address--which may be variable. Also, it is important to note that when using direct addressing any instruction which refers to an address between 00h and 7Fh is referring to Internal Memory. Any instruction which refers to an address between 80h and FFh is referring to the SFR control registers that control the 8051 microcontroller itself.

 The obvious question that may arise is, "If direct addressing an address from 80h through FFh refers to SFRs, how can I access the upper 128 bytes of Internal RAM that are available on the 8052?" The answer is: This can't be done using direct addressing. As stated, if  directly referred to an address of 80h through FFh then there will be referring to an SFR. However, there will be access to 8052s upper 128 bytes of RAM by using the next addressing mode, "indirect addressing".

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