EL21C Class Test #1-- 19^th October 2004 [1 hour]

Attempt ALL questions

1. What is the status of the Flag bits, after the following instructions, assuming

A = 11H, B = 22H, C = A7H, H = CCH, L = 00H, and all Flag bits are reset

before each instruction:

( a ) ADD L [1]

( b ) ORA B [1]

( c ) MOV H, L [1]

( d ) XRA B [1]

( e ) DCX C [1]

2. Write a short Assembly Language routine that ADD the contents of memory

locations 0023H & FF00H, then store the result in 2000H. [4]

3. How would you determine where the Stack is located ? [1]

4. The subroutine DLAY, is shown below. Determine exactly the time this routine

takes if it is entered with DE=0010h. What registers are destroyed ?

(Use Internal Clock Frequency = 3 MHz) [5]

DLAY: DCX D [ 6 states ]

MOV A,D [ 4 states ]

ORA E [ 4 states ]

JNZ DLAY [7/10 states]

RET [10 states ]

5. Figure 1 shows a 3:8 Decoder. What address ranges would enable outputs Y₀

and Y₇ ? [5]

Figure 1

6. What address does the 8085 CPU go to when it executes

the RST 4 instruction ? [1]

7. How many Ports can the 8085 CPU address ? [1]

8. MVI A,22

OUT 2

What would these two instructions accomplish on the SDK-85 lab kit ? [2]

9. Write Assembly Language code to:

(a) Setup (initialise) Ports [2]

(b) Read Port 21H switches & display status on LEDs. Which LEDs

would be ON ? [4]

10. What are the contents of the PC register on the 8085 ? [1]

11. If the 8085 CPU was performing a Write operation to memory

(eg: MOV M, D ), which Control Bus signals would be active ? [2]

12. How many Data pins does the 8085-A CPU contain ? [1]

13. How many Address pins does the 8085-B CPU contain ? [1]

Bonus: The Department of Computer Science, Mathematics & Physics has a new

sign on the outside face (eastern side) of the office building. The lettering is

White, what is the background colour ???

Maximum mark is 35.