1. Enter program
Enter the below program using Notepad or any other ASCII text editor. Add your name on the first line in the title. When saving the file, use HW1.ASM as the name. You should be able to copy and paste the following into NotePad or other word processor as an ASCII file (be certain that the extension is ASM and not TXT if saving in NotePad).
Title YOUR NAME Home Work 1 - Evaluate U = (R+6-T)*S
DATA Segment ;;Data Segment
R DW 100 ;; R = 100
S DW 8 ;; S = 8
T DW 1 ;; T = 1
U DW ? ;; U is uninitialized
DATA Ends ;; End of segment DATA
ALGORITHM Segment ;; Algorithm Segment
Assume Cs:ALGORITHM, Ds:DATA, Ss:SSeg
HW1 Proc Far ;; Procedure HW1
Mov Bx, Seg Data ;; Initialize DS register
Mov Ds, Bx ;; to address of data segment
Mov Ax, R ;; Ax = R
Add Ax, 6 ;; Ax = R+6
Sub Ax, T ;; Ax = R+6-T
Mov Dx, 1234h ;; Notice value of Dx after Mul below
Mul S ;; Dx:Ax = (R+6-T)*S
Mov U, Ax ;; U = (R+6-T)*S
Mov Ah, 4ch ;; Terminate process returning
Int 21h ;; to parent (Debugger or DOS)
HW1 Endp ;; End of procedure HW1
ALGORITHM Ends ;; End of segment ALGORITHM SSeg Segment STACK ;;Stack segment of 254 bytes
DB 254 dup(?)
SSeg Ends
End HW1 ;; End of file, start execution at HW1 |
2. Assemble
u:\user\c335\assembler Do once at command prompt.
ml /Fl hw1.asm Do each time program changed.
3. Execute
Hw1.Lst Microsoft (R) Macro Assembler Version 6.14.8444 08/15/02 15:09:01 YOUR NAME Home Work 1 - Evaluate U = (R+6-T)*S Page 1 - 1 Title YOUR NAME Home Work 1 - Evaluate U = (R+6-T)*S 0000 DATA Segment ;;Data Segment 0000 0064 R DW 100 ;; R = 100 0002 0008 S DW 8 ;; S = 8 0004 0001 T DW 1 ;; T = 1 0006 0000 U DW ? ;; U is uninitialized 0008 DATA Ends ;; End of segment DATA 0000 ALGORITHM Segment ;;Algorithm Segment Assume Cs:ALGORITHM, Ds:DATA, Ss:SSeg 0000 HW1 Proc Far ;; Procedure HW1 0000 BB ---- R Mov Bx, Seg DATA ;; Initialize DS register 0003 8E DB Mov Ds, Bx ;; to address of data segment 0005 A1 0000 R Mov Ax, R ;; Ax = R 0008 83 C0 06 Add Ax, 6 ;; Ax = R+6 000B 2B 06 0004 R Sub Ax, T ;; Ax = R+6-T 000F BA 1234 Mov Dx, 1234h ;; Notice value of Dx after Mul below 0012 F7 26 0002 R Mul S ;; Dx:Ax = (R+6-T)*S 0016 A3 0006 R Mov U, Ax ;; U = (R+6-T)*S 0019 B4 4C Mov Ah, 4ch ;; Terminate process returning 001B CD 21 Int 21h ;; to parent (Debugger or DOS) 001D HW1 Endp ;; End of procedure HW1 001D ALGORITHM Ends ;; End of segment ALGORITHM 0000 SSeg Segment STACK ;;Stack segment of 254 bytes 0000 00FE [ DB 254 dup(?) 00 ] 00FE SSeg Ends End HW1
Use the debugger to execute the Hw1.Exe file by entering:
debug hw1.exe
4. Debugging
The debugger will display a command prompt of a '-'. The four commands needed are:
- u - Unassemble the machine code. Lists the mnemonic that correspond to the machine code. ADD AX, +06 corresponds to 83C006 machine code.
- r - Register view. Displays the registers and next instruction to execute.
- p - Proceed by executing the next one or more instructions. The registers and next instruction to execute are displayed.
- d - Dump memory. The d ds:0 command dumps memory pointed to by the DS register. Memory containing the program data variables has been dumped, variable S has been located and underlined in the following example.
5. The IP register points to the NEXT instruction to be executed by the CPU at location cs:0005 in the Algorithm. To execute one instruction at a time and witness the effects on registers and memory press:
Execute instructions by pressing p until the instruction at address 0008 is displayed as below:p
12C0:0008 83C006 Add Ax, 6 Address Instruction Mnemonic |
6. Locating variables in the data segment
Variables correspond to a memory location, variable S corresponds to data segment location 0002 and 0003, since it is a word it occupies 2 bytes. The location of variable S in the data segment can be determined by examining the Hw1.Lst listing which contains the following information.
Use the debugger dump command, to examine variable S at location 0002-0003 count across the DATA pane row, starting at 0000 and counting to location 0002. Note that variable S = 0008 but is stored as 08 00 in memory backward as low high byte.:Location Value Variable
0002 0008 S DW 8 ;; S = 8
7. Continue to execute each instruction (by pressing p) until the next instruction to execute is at address 0019.
01 00 00 00 Address 0000 0001 0002 0003 0004 0005 0006 0007 Variable S
AX=0348 BX=12BF CX=002D DX=0000 SP=00FE BP=0000 SI=0000 DI=0000 DS=12BF ES=12AF SS=12C2 CS=12C0 IP=0019 NV UP EI PL NZ NA PE NC 12C0:0019 B44C MOV AH,4C |
Alt PrtSc
Paste into WordPad or other word processor to print.
The program's execution effects on data has been completed and results appear in the DATA segment. You will need to locate the memory used by each variable and determine their values. The program has not executed to completion however. Up to this point the program has been executed under the control of the debugger, executing one instruction at a time. To allow the CPU to execute the remaining instructions to completion enter:
g - Go execute program. Starts execution at the next instruction. The following executes through the last instruction Int 21h which terminates the program.
g Program terminated normally |
q - Quit debugger.
- values of variables R-U in the data segment dump circled and identified as R-U as in Step 6.
- the FINAL values of variables R-U as the corresponding decimal values (remember that words are stored in reverse byte order in memory, 123416 is stored as 34 12).
The Assembler software used in Assignment 2 and the remainder of course homeworks can be downloaded and installed by the following instructions:
Assembler documentation can be downloaded from this Web site.