Sub1 PROC
1[ ] b
LOCAL array1[50]:DWORD ; 200 bytes
Sub2 PROC
LOCAL array2[80]:WORD ; 160 bytes
Sub3 PROC
LOCAL array3[300]:WORD ; 300 bytes
What's Next
Stack Frames
Recursion
.MODEL Directive
INVOKE, ADDR, PROC, and PROTO
Creating Multimodule Programs
.MODEL Directive
.MODEL directive specifies a program's memory model and model options (language-specifier).
Syntax:
.MODEL memorymodel [,modeloptions]
memorymodel can be one of the following:
¾ tiny, small, medium, compact, large, huge, or flat
modeloptions includes the language specifier:
¾ procedure naming scheme
¾ parameter passing conventions
Memory Models
A program's memory model determines the number and sizes of code and data segments.
Real-address mode supports tiny, small, medium, compact, large, and huge models.
Protected mode supports only the flat model
Protected mode supports only the flat model.
Small model: code < 64 KB, data (including stack) <
64 KB. All offsets are 16 bits.
Flat model: single segment for code and data, up to 4 GB. All offsets are 32 bits.
Language Specifiers
C
¾ procedure arguments pushed on stack in reverse order (right to left)
¾ calling program cleans up the stack PASCAL
PASCAL
¾ procedure arguments pushed in forward order (left to right)
¾ called procedure cleans up the stack
STDCALL
¾ procedure arguments pushed on stack in reverse order (right to left)
¾ called procedure cleans up the stack
What's Next
Stack Frames
Recursion
.MODEL Directive
INVOKE, ADDR, PROC, and PROTO
Creating Multimodule Programs
INVOKE, ADDR, PROC, and PROTO
INVOKE Directive
ADDR Operator
PROC Directive
PROTO Directive
Parameter Classifications
Debugging Tips
INVOKE Directive
The INVOKE directive is a powerful
replacement for Intel’s CALL instruction that lets you pass multiple arguments
Syntax:
INVOKE procedureName [, argumentList]
ArgumentList is an optional comma-delimited list of procedure arguments
Arguments can be:
¾ immediate values and integer expressions
¾ variable names
¾ address and ADDR expressions
¾ register names
INVOKE Examples
.data
byteVal BYTE 10 wordVal WORD 1000h .code
; direct operands:
INVOKE Sub1,byteVal,wordVal i
; address of variable:
INVOKE Sub2,ADDR byteVal
; register name, integer expression:
INVOKE Sub3,eax,(10 * 20)
; address expression (indirect operand):
INVOKE Sub4,[ebx]
INVOKE Example
.data
val1 DWORD 12345h val2 DWORD 23456h .code
INVOKE AddTwo val1 val2 INVOKE AddTwo, val1, val2
push val1 push val2 call AddTwo
ADDR Operator
• Returns a near or far pointer to a variable, depending on which memory model your program uses:
• Small model: returns 16-bit offset
• Large model: returns 32-bit segment/offset
• Flat model: returns 32-bit offset
.data
myWord WORD ? .code
INVOKE mySub,ADDR myWord
• Simple example:
Your Turn . . .
Create a procedure named Difference that
subtracts the first argument from the second one.
Following is a sample call:
push 14 ; first argument
push 30p ; second argument; g
call Difference ; EAX = 16
Difference PROC push ebp
mov ebp,esp
mov eax,[ebp + 8] ; second argument sub eax,[ebp + 12] ; first argument pop ebp
ret 8
Difference ENDP
Passing by Value
When a procedure argument is passed by value, a copy of a 16-bit or 32-bit integer is pushed on the stack.
Example:
.data
myData WORD 1000h .code
main PROC
INVOKE Sub1, myData
push myData call Sub1
MASM generates the following code:
Passing by Reference
When an argument is passed by reference, its address is pushed on the stack. Example:
.data
myData WORD 1000h .code
main PROC
INVOKE Sub1, ADDR myData
push OFFSET myData call Sub1
MASM generates the following code:
PROC Directive
The PROC directive declares a procedure with an optional list of named parameters.
Syntax:
label PROC paramList
paramList is a list of parameters separated by
paramList is a list of parameters separated by
commas. Each parameter has the following syntax:
paramName : type
type must either be one of the standard ASM types
(BYTE, SBYTE, WORD, etc.), or it can be a pointer
to one of these types.
PROC Directive (cont.)
Alternate format permits parameter list to be on one or more separate lines:
label PROC, paramList
Th t b th li
comma required
The parameters can be on the same line . . .
param-1:type-1, param-2:type-2, . . ., param-n:type-n
Or they can be on separate lines:
param-1:type-1, param-2:type-2, . . .,
param-n:type-n
PROC Examples
FillArray PROC,
pArray:PTR BYTE, fillVal:BYTE arraySize:DWORD
FillArray receives a pointer to an array of bytes, a
single byte fill value that will be copied to each element of the array, and the size of the array.
y
mov ecx,arraySize mov esi,pArray mov al,fillVal L1: mov [esi],al
inc esi loop L1 ret
FillArray ENDP
PROC Examples (cont.)
Swap PROC,
pValX:PTR DWORD, pValY:PTR DWORD . . .
Swap ENDP
ReadFile PROC,
pBuffer:PTR BYTE
LOCAL fileHandle:DWORD . . .
ReadFile ENDP
PROTO Directive
Creates a procedure prototype
Syntax:
¾ label PROTO paramList
Every procedure called by the INVOKE directive y p y must have a prototype
A complete procedure definition can also serve
as its own prototype
PROTO Directive
Standard configuration: PROTO appears at top of the program listing, INVOKE appears in the code segment, and the procedure implementation occurs later in the program:
MySub PROTO ; procedure prototype MySub PROTO ; procedure prototype .code
INVOKE MySub ; procedure call
MySub PROC ; procedure implementation .
.
MySub ENDP
PROTO Example
Prototype for the ArraySum procedure, showing its parameter list:
ArraySum PROTO,
ptrArray:PTR DWORD, ; points to the array szArray:DWORD ; array size
szArray:DWORD ; array size
WriteStackFrame Procedure
Displays contents of current stack frame
¾ Prototype:
WriteStackFrame PROTO,
numParam:DWORD, ; number of passed parameters numLocalVal: DWORD ; number of DWordLocal variables numLocalVal: DWORD, ; number of DWordLocal variables numSavedReg: DWORD ; number of saved registers
WriteStackFrame Example
main PROC
mov eax, 0EAEAEAEAh
mov ebx, 0EBEBEBEBh
INVOKE aProc, 1111h, 2222h
exit
main ENDP
aProc PROC USES eax ebx,
x: DWORD, y: DWORD
LOCAL a:DWORD, b:DWORD
PARAMS = 2
LOCALS = 2
SAVED_REGS = 2
mov a,0AAAAh
mov b,0BBBBh
INVOKE WriteStackFrame, PARAMS, LOCALS, SAVED_REGS
Parameter Classifications
An input parameter is data passed by a calling program to a procedure.
¾ The called procedure is not expected to modify the
corresponding parameter variable, and even if it does, the modification is confined to the procedure itself.
A t t t i t d b i i t t
• An input-output parameter is a pointer to a variable containing input that will be both used and modified by the procedure.
• The variable passed by the calling program is modified.
• An output parameter is created by passing a pointer to a variable when a procedure is called.
• The procedure does not use any existing data from the variable, but it fills in a new value before it returns.
Example: Exchanging Two Integers
Swap PROC USES eax esi edi,
pValX:PTR DWORD, ; pointer to first integer
The Swap procedure exchanges the values of two 32-bit integers. pValX and pValY do not change values, but the integers they point to are modified.
p , ; p g
pValY:PTR DWORD ; pointer to second integer mov esi,pValX ; get pointers
mov edi,pValY
mov eax,[esi] ; get first integer xchg eax,[edi] ; exchange with second mov [esi],eax ; replace first integer ret
Swap ENDP
Trouble-Shooting Tips
Save and restore registers when they are modified by a procedure.
¾ Except a register that returns a function result
• When using INVOKE, be careful to pass a pointer to the correct data type.
• For example, MASM cannot distinguish between a DWORD argument and a PTR BYTE argument.
• Do not pass an immediate value to a procedure that expects a reference parameter.
• Dereferencing its address will likely cause a general-protection fault.
What's Next
Stack Frames
Recursion
.MODEL Directive
INVOKE, ADDR, PROC, and PROTO
Creating Multimodule Programs
Multimodule Programs
A multimodule program is a program whose source code has been divided up into
separate ASM files.
Each ASM file (module) is assembled into a separate OBJ file
separate OBJ file.
All OBJ files belonging to the same program are linked using the link utility into a single EXE file.
¾ This process is called static linking
Advantages
Large programs are easier to write, maintain, and debug when divided into separate source code modules.
• When changing a line of code, only its enclosing module needs to be assembled again. Linking assembled modules requires little time.
• A module can be a container for logically related code and data (think object-oriented here...)
• encapsulation: procedures and variables are automatically hidden in a module unless you declare them public
Creating a Multimodule Program
Here are some basic steps to follow when creating a multimodule program:
¾ Create the main module
¾ Create a separate source code module for each procedure or set of related procedures each procedure or set of related procedures
¾ Create an include file that contains procedure prototypes for external procedures (ones that are called between modules)
¾ Use the INCLUDE directive to make your procedure prototypes available to each module
Example: ArraySum Program
Summation Program (main)
Clrscr PromptForIntegers ArraySum DisplaySum
WriteString
WriteString ReadInt WriteIntWriteInt
Each of the four white rectangles will become a module.
Sample Program output
Enter a signed integer: -25 Enter a signed integer: 36 Enter a signed integer: 42
The sum of the integers is: +53
INCLUDE File
INCLUDE Irvine32.inc
PromptForIntegers PROTO,
ptrPrompt:PTR BYTE, ; prompt string
The sum.inc file contains prototypes for external functions that are not in the Irvine32 library:
ptrArray:PTR DWORD, ; points to the array arraySize:DWORD ; size of the array ArraySum PROTO,
ptrArray:PTR DWORD, ; points to the array count:DWORD ; size of the array DisplaySum PROTO,
ptrPrompt:PTR BYTE, ; prompt string theSum:DWORD ; sum of the array
Main.asm
TITLE Integer Summation Program INCLUDE sum.inc
.code
main PROC
call Clrscr
INVOKE PromptForIntegers, ADDR prompt1,
ADDR array, Count
...
call Crlf
INVOKE ExitProcess,0 main ENDP
END main