Things I learned from grading HW2

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Things I learned from grading HW2

• a leading 0 tells a86 to interpret the number as
  hexMov al, 27 al ? 2710Mov al, 027 al ?
  2716

Things to be careful of

• A program is not debugged by checking a single
  case
• Dont assume that registers are zero unless you
  set them to zero

2
About comments

• This is NOT a comment, it is redundantCmp AH,
  0 AH - 0, result not storedJE L1 jump to
  L1 if AH 0
• A comment conveys extra information Cmp AH, 0
  If the remainder is zero, then itsJE L1 an
  integer multiple of 39--so quit

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Homework for Thursday, September 2

• Write, assemble, and debug an 8088 assembly
  language program which includes the following
  statements
• Numbytes db 9
• Array db 1,2,2,3,3,3,4,4,5
• Target db 2
• Count db ?
• The program should use a loop with indexed
  addressing to count the number of occurrences of
  target in Array.
• Debug your program by changing the value of
  Target in the debugger and making sure the
  program works. Be sure to try a target which is
  not included in Array.

4
Flow Chart
5
Program to solve HW
Jmp gt L1 Numbytes db 9 Array db
1,2,2,3,3,3,4,4,5 Target db 2 Count db
? L1 Mov Ch,0 Mov CL, Numbytes CX set to go
through Array Mov Count, 0 count number of
hits Mov Si, 0 for indexed addressing Mov
Bl, target to avoid memory/memory
instructions L2 Cmp Array Si,Bl jne gt
l3 if not equal, dont increment inc
count l3 inc si point to next byte Loop
L2 l4 a destination for g command
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8088 Addressing

• 20 Address/Data lines access 8088 memory
• CS, DS, ES, SS are the four 16-bit internal
  segment registers
• These are combined with another 16-bit register
  to generate the 20-bit address
• CSIP are combined to point to the next
  instruction/data in memory.Effective address
  CS 16 IP CS 1016 IP
• CS A100, IP 1234, Effective address is A1000
  1234 A2234

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.com vs .exe

• A com file is created when CS DS ES SS
• All parts of the program fit within 64K
• (64K is the address range of a single 16-bit
  register, like IP)
• An exe file is created when two different
  memory segments are used.

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The stack

• The current stack location is defined by SSSP
• The stack is used for some operations like CALL
  and RET
• Programmers may use the stack as scratchpad
  memory
• PUSH puts a word onto the stack
• POP pulls a word from the stack
• The stack is a LIFO (Last-in, First-Out) register

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Stack Pointer

• The stack pointer, SP, always points to the most
  recently filled location in the stack
• The SP is initialized at the bottom of the
  segmentAs words are added to the stack, the
  stack fills up towards the beginning of the
  segment.
• Thus, after a PUSH instruction, SP ? SP -2after
  a POP instruction, SP ? SP 2
• 8088 assembly language allows only one word-wide
  operand to Push and PopA86 allows multiple
  operands and converts them into multiple
  instructions at the time of assembly

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8088 Procedures (Subroutines)

• Call dest
• Address of next instruction is pushed onto the
  stack
• Therefore, SP ? SP-2
• IP ? dest
• RET (no operand)
• IP ? return address from the stack
• Therefore, SP ? SP 2

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Three ways to crash your computer

• The RET instruction takes the address of the next
  opcode from the stack
• If this address is wrong, the results are
  unpredictable
• Within a procedure, Push and Pop instructions
  must occur in pairs
• CRASH, if inside a procedure
• the number of Pushes exceeds the number of Pops,
  or
• the number of Pops exceeds the number of Pushes,
  or
• the program changes the SP and does not restore
  it to its original value before RET

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Push and Pop and Procedures

• Push and Pop are used to preserve registers when
  a procedure is called

Push Ax, BxCall DOITPop Bx, AX DOit
manipulations involving AX and BXRET
Call ADDIT Addit Push Ax, Bx
manipulations involving AX and BX
Pop Bx, AX RET
vs
Which is better programming?
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Good Programming

• A procedure should return with all registers
  unchanged, except for registers used for output!

Call ADDIT Addit Push Ax, Bx
manipulations involving AX and BX
Pop Bx, AX RET
As a practical matter, otherwise the programmer
must remember to Push and Pop every time the
procedure is called.
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CallRet.485
Use this program and debugger to understand Call,
Ret, Push, and Pop
mov al, 3 mov bl, 5 call mulab mulab sets cx
albl and doesn't change al or bl mov
dx,cx mulab push ax,bx to save registers mul
bl mov cx,ax pop bx,ax to restore registers ret
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a86 switches and DOS

• Path c\ c\dos c\a86 c\a86\a86lib c\util
• The path statement tells dos what directories to
  look in when its searching for an executable
  file(.com, .exe, .bat)
• set a86P64
• this tells a86 to create machine language
  instructions for the 8088 instruction
  setOtherwise a86 will assemble for the
  computers processor
• set a86libc\a86\a86lib
• When a86 encounters a symbol that it does not
  know, it looks in the library. This switch tells
  a86 where to find the library

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a86 libraries

• Step 1) Create a useful procedure.
• Make sure labels and variable names are unique
• Procedures should primarily use local labels
• Step 2) a86lib filename.ext
• a86lib.com is a registered-user-only executable
  file that adds the procedure to the library
  indexer, a86.lib
• Step 3) at assembly, when a86 encounters a
  procedure name that it does not know, it looks to
  a86.lib (the set a86lib switch tells it where to
  look).
• If the procedure is found there, the ASCII text
  of the procedure is appended to the program being
  assembled
• Step 4) a86 assembles the program the appended
  procedures into a machine-language executable
• This is different than a mainframe executable
  file library.

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licensing considerations

• a86 and d86 are Shareware. Anyone may download
  and test a86 and d86 to determine if it will meet
  his or her needs. Someone who decides to use it
  is obligated to pay the fee and license the
  product.
• The University of Mississippi has purchased a
  site license for a86/d86. a86, d86, and
  a86lib.com can be used on any university-owned
  computer.
• a86lib.com is restricted to licensed users only.
  Possession or use of a86lib.com outside the
  university is a violation of the law, unless the
  user has paid the license fee.
• Libraries created with a86lib.com (the a86.lib
  file) are not licensed and may be distributed.
  Executables may be distributed.

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EE 485/486 and libraries

• Unless specifically prohibited, you can assume
  the standard library procedures are present when
  writing any homework assignment or answering any
  test question in EE 485 or EE 486