Question

in assembly

1. Real mode allows access to memory below which memory address?

2. Under real and protected mode, the far jump modifies the current instruction address by changing which register or registers?

3. Under real and protected mode, the near jump modifies the current instruction address by changing which register or registers?

4. Under real mode, briefly explain what happens when the PUSH BX instruction executes. Make sure to show where BH and BL are stored. Assume that initially SP=0100H and SS=0200H. Recall that in the x86 family, the stack grown downwards, toward lower addresses, and the SP is pointing the to top of the stack (most recently pushed value).

Answer 1

1. - Real mode gives exactly 1MiB of addressable memory and direct software access to ALL ADDRESSABLE MEMORY as well ass peripheral devices.

2. - In far jump - no flag is modified. labelled as - JMP label, and it updates CS and IP.

3. - in near jump - no flag is modified, labelled as - JMP label, and updates IP only.

4. - In real mode, while executing PUSH instruction, always remember that the instructions follows PREDECREMENT operation by 2 steps. i.e.

if SP initially at 0100H then after execution of the PUSh instruction, 00FE H will be the address where we will observe SP after the execution. and every time the SP will be at the top of the stack only.

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plus to this - stack grows downwards only when POP is executed and upwards when PUSH is executed.