CHAPTER ELEVEN: PROCEDURES AND FUNCTIONS (Part 1)


CHAPTER ELEVEN: PROCEDURES AND FUNCTIONS (Part 1)

11.0 - Chapter Overview 11.1 - Procedures 11.2 - Near and Far Procedures 11.2.1 - Forcing NEAR or FAR CALLs and Returns 11.2.2 - Nested Procedures 11.3 - Functions 11.4 - Saving the State of the Machine 11.5 - Parameters 11.5.1 - Pass by Value 11.5.2 - Pass by Reference 11.5.3 - Pass by Value-Returned 11.5.4 - Pass by Result 11.5.5 - Pass by Name 11.5.6 - Pass by Lazy-Evaluation 11.5.7 - Passing Parameters in Registers 11.5.8 - Passing Parameters in Global Variables 11.5.9 - Passing Parameters on the Stack 11.5.10 - Passing Parameters in the Code Stream 11.5.11 - Passing Parameters via a Parameter Block 11.6 - Function Results 11.6.1 - Returning Function Results in a Register 11.6.2 - Returning Function Results on the Stack 11.6.3 - Returning Function Results in Memory Locations 11.7 - Side Effects 11.8 - Local Variable Storage 11.9 - Recursion 11.10 - Sample Program		Copyright 1996 by Randall Hyde All rights reserved. Duplication other than for immediate display through a browser is prohibited by U.S. Copyright Law. This material is provided on-line as a beta-test of this text. It is for the personal use of the reader only. If you are interested in using this material as part of a course, please contact rhyde@cs.ucr.edu Supporting software and other materials are available via anonymous ftp from ftp.cs.ucr.edu. See the "/pub/pc/ibmpcdir" directory for details. You may also download the material from "Randall Hyde's Assembly Language Page" at URL: http://webster.ucr.edu Notes: This document does not contain the laboratory exercises, programming assignments, exercises, or chapter summary. These portions were omitted for several reasons: either they wouldn't format properly, they contained hyperlinks that were too much work to resolve, they were under constant revision, or they were not included for security reasons. Such omission should have very little impact on the reader interested in learning this material or evaluating this document. This document was prepared using Harlequin's Web Maker 2.2 and Quadralay's Webworks Publisher. Since HTML does not support the rich formatting options available in Framemaker, this document is only an approximation of the actual chapter from the textbook. If you are absolutely dying to get your hands on a version other than HTML, you might consider having the UCR Printing a Reprographics Department run you off a copy on their Xerox machines. For details, please read the following EMAIL message I received from the Printing and Reprographics Department: Hello Again Professor Hyde, Dallas gave me permission to take orders for the Computer Science 13 Manuals. We would need to take charge card orders. The only cards we take are: Master Card, Visa, and Discover. They would need to send the name, numbers, expiration date, type of card, and authorization to charge $95.00 for the manual and shipping, also we should have their phone number in case the company has any trouble delivery. They can use my e-mail address for the orders and I will process them as soon as possible. I would assume that two weeks would be sufficient for printing, packages and delivery time. I am open to suggestions if you can think of any to make this as easy as possible. Thank You for your business, Kathy Chapman, Assistant Printing and Reprographics University of California Riverside (909) 787-4443/4444 We are currently working on ways to publish this text in a form other than HTML (e.g., Postscript, PDF, Frameviewer, hard copy, etc.). This, however, is a low-priority project. Please do not contact Randall Hyde concerning this effort. When something happens, an announcement will appear on "Randall Hyde's Assembly Language Page." Please visit this WEB site at http://webster.ucr.edu for the latest scoop. Redesigned 10/2000 with "MS FrontPage 98" using 17" monitor 1024x768 (c) 2000 BIRCOM Entertainment'95

Modular design is one of the cornerstones of structured programming. A modular program contains blocks of code with single entry and exit points. You can reuse well written sections of code in other programs or in other sections of an existing program. If you reuse an existing segment of code, you needn't design, code, nor debug that section of code since (presumably) you've already done so. Given the rising costs of software development, modular design will become more important as time passes.

The basic unit of a modular program is the module. Modules have different meanings to different people, herein you can assume that the terms module, subprogram, subroutine, program unit, procedure, and function are all synonymous.

The procedure is the basis for a programming style. The procedural languages include Pascal, BASIC, C++, FORTRAN, PL/I, and ALGOL. Examples of non-procedural languages include APL, LISP, SNOBOL4 ICON, FORTH, SETL, PROLOG, and others that are based on other programming constructs such as functional abstraction or pattern matching. Assembly language is capable of acting as a procedural or non-procedural language. Since you're probably much more familiar with the procedural programming paradigm this text will stick to simulating procedural constructs in 80x86 assembly language.

This chapter presents an introduction to procedures and functions in assembly language. It discusses basic principles, parameter passing, function results, local variables, and recursion. You will use most of the techniques this chapter discusses in typical assembly language programs. The discussion of procedures and functions continues in the next chapter; that chapter discusses advanced techniques that you will not commonly use in assembly language programs. The sections below that have a "*" prefix are essential. Those sections with a "o" discuss advanced topics that you may want to put off for a while.

In a procedural environment, the basic unit of code is the procedure. A procedure is a set of instructions that compute some value or take some action (such as printing or reading a character value). The definition of a procedure is very similar to the definition of an algorithm. A procedure is a set of rules to follow which, if they conclude, produce some result. An algorithm is also such a sequence, but an algorithm is guaranteed to terminate whereas a procedure offers no such guarantee.

Most procedural programming languages implement procedures using the call/return mechanism. That is, some code calls a procedure, the procedure does its thing, and then the procedure returns to the caller. The call and return instructions provide the 80x86's procedure invocation mechanism. The calling code calls a procedure with the call instruction, the procedure returns to the caller with the ret instruction. For example, the following 80x86 instruction calls the UCR Standard Library sl_putcr routine[1]:

sl_putcr prints a carriage return/line feed sequence to the video display and returns control to the instruction immediately following the

call
sl_putcr

instruction.

Alas, the UCR Standard Library does not supply all the routines you will need. Most of the time you'll have to write your own procedures. A simple procedure may consist of nothing more than a sequence of instructions ending with a ret instruction. For example, the following "procedure" zeros out the 256 bytes starting at the address in the bx register:

By loading the bx register with the address of some block of 256 bytes and issuing a call ZeroBytes instruction, you can zero out the specified block.

As a general rule, you won't define your own procedures in this manner. Instead, you should use MASM's proc and endp assembler directives. The ZeroBytes routine, using the proc and endp directives, is

Keep in mind that proc and endp are assembler directives. They do not generate any code. They're simply a mechanism to help make your programs easier to read. To the 80x86, the last two examples are identical; however, to a human being, latter is clearly a self-contained procedure, the other could simply be an arbitrary set of instructions within some other procedure. Consider now the following code:

Are there two procedures here or just one? In other words, can a calling program enter this code at labels ZeroBytes and DoFFs or just at ZeroBytes? The use of the proc and endp directives can help remove this ambiguity:

Always keep in mind that the proc and endp directives are logical procedure separators. The 80x86 microprocessor returns from a procedure by executing a ret instruction, not by encountering an endp directive. The following is not equivalent to the code above:

Without the ret instruction at the end of each procedure, the 80x86 will fall into the next subroutine rather than return to the caller. After executing ZeroBytes above, the 80x86 will drop through to the DoFFs subroutine (beginning with the mov cx, 128 instruction). Once DoFFs is through, the 80x86 will continue execution with the next executable instruction following DoFFs' endp directive.

The near or far operand is optional, the next section will discuss its purpose. The procedure name must be on the both proc and endp lines. The procedure name must be unique in the program.

Every proc directive must have a matching endp directive. Failure to match the proc and endp directives will produce a block nesting error.

The 80x86 supports near and far subroutines. Near calls and returns transfer control between procedures in the same code segment. Far calls and returns pass control between different segments. The two calling and return mechanisms push and pop different return addresses. You generally do not use a near call instruction to call a far procedure or a far call instruction to call a near procedure. Given this little rule, the next question is "how do you control the emission of a near or far call or ret?".

Unfortunately, these instructions do not tell MASM if you are calling a near or far procedure or if you are returning from a near or far procedure. The proc directive handles that chore. The proc directive has an optional operand that is either near or far. Near is the default if the operand field is empty[3]. The assembler assigns the procedure type (near or far) to the symbol. Whenever MASM assembles a call instruction, it emits a near or far call depending on operand. Therefore, declaring a symbol with proc or proc near, forces a near call. Likewise, using proc far, forces a far call.

Besides controlling the generation of a near or far call, proc's operand also controls ret code generation. If a procedure has the near operand, then all return instructions inside that procedure will be near. MASM emits far returns inside far procedures.

Once in a while you might want to override the near/far declaration mechanism. MASM provides a mechanism that allows you to force the use of near/far calls and returns.

Use the near ptr and far ptr operators to override the automatic assignment of a near or far call. If NearLbl is a near label and FarLbl is a far label, then the following call instructions generate a near and far call, respectively:

Suppose you need to make a far call to NearLbl or a near call to FarLbl. You can accomplish this using the following instructions:

Calling a near procedure using a far call, or calling a far procedure using a near call isn't something you'll normally do. If you call a near procedure using a far call instruction, the near return will leave the cs value on the stack. Generally, rather than:

Calling a far procedure with a near call is a very dangerous operation. If you attempt such a call, the current cs value must be on the stack. Remember, a far ret pops a segmented return address off the stack. A near call instruction only pushes the offset, not the segment portion of the return address.

Starting with MASM v5.0, there are explicit instructions you can use to force a near or far ret. If ret appears within a procedure declared via proc and end;, MASM will automatically generate the appropriate near or far return instruction. To accomplish this, use the retn and retf instructions. These two instructions generate a near and far ret, respectively.

MASM allows you to nest procedures. That is, one procedure definition may be totally enclosed inside another. The following is an example of such a pair of procedures:

Unlike some high level languages, nesting procedures in 80x86 assembly language doesn't serve any useful purpose. If you nest a procedure (as with InsideProc above), you'll have to code an explicit jmp around the nested procedure. Placing the nested procedure after all the code in the outside procedure (but still between the outside proc/endp directives) doesn't accomplish anything. Therefore, there isn't a good reason to nest procedures in this manner.

Whenever you nest one procedure within another, it must be totally contained within the nesting procedure. That is, the proc and endp statements for the nested procedure must lie between the proc and endp directives of the outside, nesting, procedure. The following is not legal:

The OutsideProc and InsideProc procedures overlap, they are not nested. If you attempt to create a set of procedures like this, MASM would report a "block nesting error". The figure below demonstrates this graphically:

Besides fitting inside an enclosing procedure, proc/endp groups must fit entirely within a segment. Therefore the following code is illegal:

The endp directive must appear before the

cseg
ends

statement since MyProc begins inside cseg. Therefore, procedures within segments must always take the form shown below:

Not only can you nest procedures inside other procedures and segments, but you can nest segments inside other procedures and segments as well. If you're the type who likes to simulate Pascal or C procedures in assembly language, you can create variable declaration sections at the beginning of each procedure you create, just like Pascal:

ZeroWords follows the main program because it belongs to a different segment (cseg2) than MainPgm (cseg1). Remember, the assembler and linker combine segments with the same class name into a single segment before loading them into memory (see Chapter Eight for more details). You can use this feature of the assembler to "pseudo-Pascalize" your code in the fashion shown above. However, you'll probably not find your programs to be any more readable than using the straight forward non-nesting approach.

The difference between functions and procedures in assembly language is mainly a matter of definition. The purpose for a function is to return some explicit value while the purpose for a procedure is to execute some action. To declare a function in assembly language, use the proc/endp directives. All the rules and techniques that apply to procedures apply to functions. This text will take another look at functions later in this chapter in the section on function results. From here on, procedure will mean procedure or function.

This section of code attempts to print ten lines of 40 spaces each. Unfortunately, there is a subtle bug that causes it to print 40 spaces per line in an infinite loop. The main program uses the loop instruction to call PrintSpaces 10 times. PrintSpaces uses cx to count off the 40 spaces it prints. PrintSpaces returns with cx containing zero. The main program then prints a carriage return/line feed, decrements cx, and then repeats because cx isn't zero (it will always contain 0FFFFh at this point).

The problem here is that the PrintSpaces subroutine doesn't preserve the cx register. Preserving a register means you save it upon entry into the subroutine and restore it before leaving. Had the PrintSpaces subroutine preserved the contents of the cx register, the program above would have functioned properly.

Use the 80x86's push and pop instructions to preserve register values while you need to use them for something else. Consider the following code for

Note that PrintSpaces saves and restores ax and cx (since this procedure modifies these registers). Also, note that this code pops the registers off the stack in the reverse order that it pushed them. The operation of the stack imposes this ordering.

Either the caller (the code containing the call instruction) or the callee (the subroutine) can take responsibility for preserving the registers. In the example above, the callee preserved the registers. The following example shows what this code might look like if the caller preserves the registers:

There are two advantages to callee preservation: space and maintainability. If the callee preserves all affected registers, then there is only one copy of the push and pop instructions, those the procedure contains. If the caller saves the values in the registers, the program needs a set of push and pop instructions around every call. Not only does this make your programs longer, it also makes them harder to maintain. Remembering which registers to push and pop on each procedure call is not something easily done.

On the other hand, a subroutine may unnecessarily preserve some registers if it preserves all the registers it modifies. In the examples above, the code needn't save ax. Although PrintSpaces changes the al, this won't affect the program's operation. If the caller is preserving the registers, it doesn't have to save registers it doesn't care about:

This example provides three different cases. The first loop (Loop0) only preserves the cx register. Modifying the al register won't affect the operation of this program. Immediately after the first loop, this code calls PrintSpaces again. However, this code doesn't save ax or cx because it doesn't care if PrintSpaces changes them. Since the final loop (Loop1) uses ax and cx, it saves them both.

One big problem with having the caller preserve registers is that your program may change. You may modify the calling code or the procedure so that they use additional registers. Such changes, of course, may change the set of registers that you must preserve. Worse still, if the modification is in the subroutine itself, you will need to locate every call to the routine and verify that the subroutine does not change any registers the calling code uses.

Preserving registers isn't all there is to preserving the environment. You can also push and pop variables and other values that a subroutine might change. Since the 80x86 allows you to push and pop memory locations, you can easily preserve these values as well.

[1] Normally you would use the putcr macro to accomplish this, but this call instruction will accomplish the same thing.

[3] Unless you are using MASM's simplified segment directives. See the appendices for details.

The Art of ASSEMBLY LANGUAGE PROGRAMMING

Chapter Ten	Table of Content	Chapter Eleven (Part 2)