Question

ECE3166 ADVANCED MICROPROCESSORS JUNE 2020 Question 2 (a) State the addressing mode and physical address of the source operand for the x86 instructions below. Then, state the results and affected registers after execution of the instructions. The initial register / memory contents are given in Table Q2. MOV ECX ES [BX][SI] - 7 [4 marks] MUL BYTE PTR[202H] [5 marks] Table Q2 Register Register Content (Hex) CS 3000 DS 3001 ES 3002 SS 3003 AX BX SI DI Content (Hex) 9214 099F 4000 CD38 Content (Hex) Content (Hex) Memory Content address (Hex) (Hex) 349BF 11 349BE 22 349BD 33 349BC 44 Memory Content address (Hex) (Hex) 349BB 55 349BA 66 349B9 77 349B8 88 Memory address (Hex) 34013 34012 34011 34010 80 26 02 Memory address (Hex) 30213 30212 30211 30210 ОВ 1D 52 61 IC (1) AL BL, CL, and DL contain unpacked BCD numbers

Answer 1

Greetings!!

i) MOV ECX,ES:[BX][SI]-7

• Addressing mode used is Based Indexed Relative addressing
• Physical address calculated:349B8H
• Register content after the execution of the instruction: 55667788H

This instruction will calculate the physical address as follows:

Physical address=Shifted content of ES+Content of BX+Content of SI-7

=30020+099F+4000-7

=349B8H[Please note that the segment register is shifted 4 position left ie 3002 become 30020]

4 bytes from this address is copied into ECX register as follows:55667788H. Here little endian scheme is used to retrieve the data. ie lower address data will move to lower part of the register and so on. Here 88 is stored in the lower address and that is copied into the lower part of the register, then the next byte and so on. Finally 55 is at the higher address and that is copied into the higher part of the register ECX.

ii) MUL BYTE PTR[202H]

• Addressing mode used is Relative addressing mode since a relative address 202 is mentioned as part of the instruction and this will the offset from the data segment.
• Physical address is 30212H
• Byte read from the memory is:1D

This instruction will calculate the physical address as follows:

Physical address=Shifted content of DS+Immediate offset

=30010+202

=30212H

One byte of data from the address 32012 is read and multiply with the content of AX and store the result back to the AX register. Answer is 108C44H and lower 16 bit of the answer 8C44 will be in regisgter AX and the remaining will be stored automatically into register DX. ie DX=0010

Hope this helps

Thank You