explain LDX D in microprocessor
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IT SHOULD BE EXPLAIN LDAX IN MICROPROCESSOR ?
A N S W E R IS ,
LDA stands for load Accumulator direct, and by its name you can tell that it uses direct addressing. Direct addressing, put simply, uses a memory location that is stored as an immediate. For instance, suppose I have a global variable named ERROR, which I declared in an assembly file using ERROR EQU 7FFFH. Then say I want to get the contents of that variable - I can use LDA ERROR or, equivalently, LDA 7FFFH. The two bytes following the opcode is the direct address.
On the other hand, LDAX stands for LoaD Accumulator indirect from register pair (X), and again by the name, you can tell that it uses indirect addressing. Indirect addressing uses the contents of register(s) as a memory address. (You can say the register is an indirection, if you’ve encountered that term before.) This is useful when, for instance, you do not know at the time when you write a program what memory location you want to access. There are only two forms of the LDAX instruction: LDAX B and LDAX D, which use the BC and DE register pairs, respectively, to address memory. If the HL register pair is desired as the memory pointer, simply use MOV A, M.
Notice that anything that can be accomplished with LDA can also be done with LDAX (with a few more instructions): for example,
LDA 7FFFH
and
LXI B, 7FFFH
LDAX B
are equivalent.
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