CS2011, A Term 1999
Prof. Sergio A. Alvarez
Practice Problems for Test 2

Name: ______________________  Login: _____________ Section: ____


Instructions: 
Read each problem carefully. Write your answers clearly. Include 
brief explanations for your solutions. Unexplained answers may 
not receive full credit. Each problem is worth 50 points.     
Good luck!



1) Assume that the two instructions PUSH BX, PUSH CX were 
executed (in that order) immediately before the following debug 
display was produced:

AX=3520  BX=00FF  CX=1234  DX=FFE2  SP=FF00  BP=0000  SI=0000  DI=0000  
DS=1000  ES=0FF0  SS=12EB  CS=1F76  IP=010A   NV UP EI PL NZ NA PO NC 
1F76:010A 58            POP AX

a) Give the new contents of the CS, IP, SS, and SP registers 
immediately after the next instruction is executed.

b) Draw a diagram that shows how the top of the stack changes as 
a result of executing the next instruction. Give explicit 
segment:offset addresses and hex contents for each memory 
location involved. Include both the old and new stack tops in 
your diagram.


2) Assume that the initial register contents are exactly as 
given above in problem 1), except that the instruction about to 
be executed is now: 

1F76:010A 082300        CALL 0130

a) Give the new contents of the CS, IP, SS, and SP registers 
immediately after the next instruction is executed.

b) Draw a diagram that shows how the top of the stack changes 
as a result of executing the next instruction. Give explicit 
segment:offset addresses and hex contents for each memory 
location involved. Include both the old and new stack tops in 
your diagram.


3) Consider the following assembly language instructions:  

     mov   ax,0103h
     mov   dx,0         
     mov   bx,0020h
     div   bx           

a) State what registers the dividend (numerator), divisor 
(denominator), quotient, and remainder will be taken from or 
placed in (in connection with the div bx instruction shown).

b) Give the final hex contents of registers AX, BX, DX after the 
above sequence of instructions has been executed.


4) Fill in the steps below to write an 80X86 assembly language 
procedure named twopower that accepts a singleword unsigned 
integer n as input on the stack and recursively calculates the 
n-th power of 2, returning the result in register AX (assume 
that the result occupies no more than 16 bits). Your answer to 
part (a) should be in high-level functional notation like 
twopower(n) = twopower(n-5). Your answers to parts (b)-(e) 
should be in 80X86 assembly language. 

a) Let twopower(n) denote the n-th power of 2. Give a recursive 
definition of twopower(n), in terms of a recursive relation 
together with a suitable base case. 

b) Define the start of a procedure named twopower, set up a 
stack frame in the usual way, and retrieve the parameter n from 
the stack, placing it in register BX.

c) Check for the base case, jump to a label called recurse if 
the base case does not occur, and handle the base case 
otherwise, jumping to a label called cleanup at the end.

d) Starting with the label recurse, handle the recursive case, 
calling twopower with the appropriate arguments, and leaving the 
desired final result in register AX.

e) Starting with the label cleanup, finalize the stack frame and 
return, clearing the stack as appropriate.


5) Given the data allocation:

   .data
     bitbunch dw  0010h
     number   dw  000Ah

Consider the following assembly language loop intended to 
accumulate in AX the product of (the low bytes of) bitbunch and 
number using the shift-and-add method described in class.

     sumloop:
        bt  bitbunch,dx
        jnc L
        add ax,number
     L: shl number,1
        inc dx
     loop sumloop

a) State explicitly what the initial value should be for each of 
the registers AX, CX, DX immediately before executing the above 
loop so that the loop correctly computes the desired product in 
AX. In each case briefly explain why. 

b) Give the correct final values for each of the registers AX, 
CX, DX immediately after the above loop has finished executing.


6)

a) Give the absolute addresses of the memory locations in which 
the interrupt vector for INT 10h is stored. Specify how many 
memory locations are needed.

b) Describe the sequence of events involved in handling software 
interrupt INT 21h. Be explicit about memory locations, control 
branching, and the like.


7) Assume the following data allocation and assume that the 
variables defined here retain their initial values. 

     .data
     buffer   db  16 dup(' ')
     xtable   db  "0123456789ABCDEF"

Write an assembly language procedure getascii that returns in AL
the ASCII code of the hex symbol (0..9,A..F) corresponding to the
number (0..15d) stored in DL. Use either the XLAT instruction or
indirect addressing to extract the ASCII code from xtable. All
registers except AX, IP, and the Flags register should be left
unchanged by a call to your procedure.