Hello there,
I have attached the assembly language code for your reference. In the first picture, you can see that I am encountering a compilation error while attempting to directly load an immediate value to a specific memory location. However, in the second picture, I am using a register to load the same value to a memory location, and there is no compilation error. Can you please explain why this is happening?
Thank you.
For this answer, please assume that the 8086 is working in segmented mode, so the programmer has 20 bits available for addresses, and you use segmentation for accessing your program, data and stack.
Note that in the first image, in the code:
MOV AX,44BAH
MOV [4000H], 7890H
ADD AX, [4000H]
INT 03H
In traditional 8086 assembly language, for the second instruction, the destination has a direct addressing mode with respect to the data segment, and the source operand is immediate value. Such an instruction does not exist in 8086. In the second image, in the code:
MOV AX,44BAH
MOV [4000H], AX
ADD AX, [4000H]
INT 03H
The second instruction does exist for 8086, the destination has a direct addressing mode with respect to the data segment, actually the 4000H is the offset with respect to the data segment register (16 bits) value with 4 zeros to the right, so you get a 20 bit address), and the source operand is a register (16 bit value).
Many assemblers accept pseudo instructions, but for the first case no assembler would have a suitable translation for MOV [4000H], 7890H into a real instruction.
Regards,
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