To generate this 20 bit physical address from 2 sixteen bit registers, the following procedure is adopted. The contents of the segment registers are shifted left four times with zeroes 0? This is similar to multiplying four hex numbers by the base The code segment registers are used to hold programs,data segment register to keep data, stack segment register for stack operations and extra segment register to keep strings of data.
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No change in the code we write that processes the file header is necessary unless you inserted an extra data element. Each time the assembler finds the given names in the program, it will replace the name with the value or a symbol. The value can be in the range 0 through and it can be another Equate declared anywhere above or below. COM file. But an Equate does not have an address or space reserved for it in the. For example: if you call a procedure which is in program module assembled at a different time from that which contains the CALL instructions ,you must tell the assembler that the procedure is external the assembler will put information in the object code file so that the linker can connect the two module together.
It is the name given to the the segment. Example: the code segment is used to indicate to the assembler the start of logical segment. It follows a name we give the procedure. This shortens the source module. An alternative is use of editor block command to cop the file into the current source module. It is used to load the offset of a variable into a register so that variable can be accessed with one of the addressed modes. This allows the contents of all he segments to be accessed from the same group.
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