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DEBUG dump interpretation practice
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idallen@ncf.ca

The following DEBUG dump excerpt is from the registers and memory given
in the final Trace and Display on the call_push_out.txt web page:

    http://www.algonquincollege.com/~alleni/dat2343/01f/call_push_out.txt

Given this DEBUG dump output:

-t
AX=0000  BX=0000  CX=0000  DX=0000  SP=FFE0  BP=0000  SI=0000  DI=0000  
DS=1426  ES=1426  SS=1426  CS=1426  IP=1234   NV UP EI PL NZ NA PO NC 
1426:1234 E84144        CALL    5678                               

-d ffe0
1426:FFE0  BF 9A 7B 56 37 12 BF 9A-7B 56 37 12 03 01 00 00   ..{V7...{V7.....
1426:FFF0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................

1.  Is this a .COM program or a .EXE program?
    How can you tell by looking at the dump?

2.  What is the Real Address of the high byte of the item on top of the
    stack at this point?
    
    (The high byte is the higher memory address of the 16-bit quantity
    on top of the stack.  The "top" of the stack is the 16-bit quantity
    addressed by the current value of SS:SP.  Note that, since the stack
    grows *downward* in memory, the "top" of the stack has an address
    *lower* than the "bottom" of the stack.  The high byte will be stored
    in higher memory than the low byte [according to Intel rules], so
    the address of the high byte will be one larger than the address of
    the low byte.)

3.  From the dump, what is the actual value (in hex) stored in memory at
    the address of the byte calculated in the previous question?

For each question below provide these three things:
 a) What is the Segment:Offset used in the final MOV instruction?
 b) What is the Real Address used in the final MOV instruction?
 c) What value would be picked up from memory and copied to register AL?

4.  MOV BP,SP
    MOV AL,[BP]

5.  MOV BX,SP
    ADD BX,1
    MOV AL,[BX]

6.  MOV BX,1446h    ; cannot move a constant directly into ES, so
    MOV ES,BX       ; we must use an intermediate BX register
    MOV BX,SP
    SUB BX,1FEh
    MOV AL,ES:[BX]  ; ASM syntax for address [ES:BX]

7.  Using the final Real Address given by ES:BX in the question above,
    and the SS:SP value from the DEBUG dump, how many bytes separate Real
    Address ES:BX from Real Address SS:SP?  How many bytes separate Real
    Address ES:0000 from Real Address SS:0000?

================================
Intel Assembly Language Practice
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8.  What is the difference between (a) and (b):

    a) MOV AH,4Ch
       MOV AL,01h
       INT 21h

    b) MOV AX,4C01h
       INT 21h

9.  What ASCII character is output on your screen after this executes?

       MOV AX,2Fh
       ADD AX,5
       MOV DL,AL
       MOV AH,02h
       INT 21h