8. (16 points) A two-word instruction is stored at location 200 with its address

Question

8. (16 points) A two-word instruction is stored at location 200 with its address field at location 201 (all numbers in hexadecimal). The first word of the instruction specifies the operation code and mode; the second word specifies the address part.

The values of the program counter (PC register), a general register (R1), the index register (XR), the base register (BR), and certain addresses in memory are as shown below.

Evaluate the effective address and the value that is loaded into the AC if the addressing mode of the instruction is:

Addressing Mode

Effective address

Value of the operand loaded into the AC

a)

Immediate

b)

Direct addressing

c)

Indirect addressing

d)

Register

e)

Register indirect

f)

Relative address

g)

Base register addressing

h)

Indexed addressing

Memory

Registers Address Content

PC =200

200

Load to AC; Mode

201

Address = 400

R1 = 900

202

Next instruction

:

:

XR = 300

300

600

:

:

BR = 200

400

300

:

:

AC = ?

500

700

:

:

600

800

:

:

602

802

:

:

700

500

:

:

800

950

:

:

900

850

Addressing Mode

Effective address

Value of the operand loaded into the AC

a)

Immediate

b)

Direct addressing

c)

Indirect addressing

d)

Register

e)

Register indirect

f)

Relative address

g)

Base register addressing

h)

Indexed addressing

Explanation / Answer

E.A value in ac a) - | 200// immediate value is taken b) 400 | 300//ea=address provided in instruction c) 300 | 600//ea=[address provided in instruction ] d) - | 900// the value in register e) 900 | 850//ea=[the value in register] f) 600 | 800 //In this mode,the operand is specified as asigned . displacement,relative to PC g) 600 | 800 //ea=br+value in instruction h) 700 | 500//ea=xr+value in istruction

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