Assembler Language Macro "Boot Camp" Part 1

Transcription

1 Assembler Language Macro "Boot Camp" Part 1 SHARE in Austin March 1-6, 2009 Session 8158 Who am I? Michael Stack, K Cats Consulting Instructor in Computer Science at Northern Illinois University from , including assembler language, data structures (in assembler), and applied systems programming Available Materials Materials described in these two sessions, as well as the Assembler Boot Camp, can be found at Today's Agenda Our First Macro Instruction The Second Version of Our ADD Macro The Macro Programming Language 1-4

2 Our First Macro Instruction In which we simply add two numbers and save the sum Recall the ADD2 Program This was first presented in the third session of the Assembler Boot Camp It adds two binary numbers located in memory and stores the result in a different memory location The version we will use is the one with labels (and USING) Demo Program from Assembler Boot Camp * This program adds two numbers that are taken * from WORD1 and WORD2 in the program. * The sum is stored in WORD3. ADD2 CSECT LR 12,15 Copy addr of 1st inst USING ADD2,12 Tell assembler L 1,WORD1 Load 1st no. into R1 L 2,WORD2 Load 2nd no. into R2 AR 1,2 Get sum in R1 ST 1,WORD3 Store sum BCR B'1111',14 Return to caller WORD1 DC F'4' Fullword initially 4 WORD2 DC F'6' Fullword initially 6 WORD3 DS F Rsrvd only, no init END ADD2 Demo Assembly from Assembler Boot Camp LOC OBJECT CODE SOURCE STATEMENT ADD2 CSECT CF LR 12, USING ADD2, C014 L 1,WORD C018 L 2,WORD A 1A12 AR 1, C 5010 C01C ST 1,WORD FE BCR B'1111', WORD1 DC F'4' WORD2 DC F'6' 00001C WORD3 DS F END ADD2 5-8

3 Our first macro instruction Notice that the "working part" of the program is the four-instruction sequence: L 1,WORD1 Load 1st no. into R1 L 2,WORD2 Load 2nd no. into R2 AR 1,2 Get sum in R1 ST 1,WORD3 Store sum We will use this sequence to create our first macro instruction From now on, we will use mixed case for instructions and operands Our first macro instruction Every macro instruction has four parts A header statement A prototype statement (right after the header) The body A trailer statement The most important part is the body, and the most important part of the body (for now) is the set of model statements These may look like assembler language statements but they are not (picky, picky) Our first macro instruction Here is our first macro definition Macro (header) Add (prototype) L 1,Word1 (model) L 2,Word2 (model) (body of AR 1,2 (model) macro) ST 1,Word3 (model) MEnd (trailer) Macros can be defined within a program (programmer macro), or can be placed in a macro library (library macro) Our first macro instruction The name of the macro is what appears in the "operation" field of the prototype statement In this case, the name is Add We use - or "invoke" - the Add macro by including in our assembler program a statement which specifies the name of the macro in the operation field 9-12

4 Source program before processing the macro instruction... Add2 CSect LR 12,15 Copy addr of 1st inst Using Add2,12 Tell assembler Add, Place sum in Word3 BCR B'1111',14 Return to caller Word1 DC F'4' Fullword initially 4 Word2 DC F'6' Fullword initially 6 Word3 DS F Rsrvd only, no init End Add2 Source program after processing the macro instruction Add2 CSect LR 12,15 Copy addr of 1st inst Using Add2,12 Tell assembler Add, Place sum in Word3 + L 1,Word1 Load 1st no. into R1 + L 2,Word2 Load 2nd no. into R2 + AR 1,2 Get sum in R1 + ST 1,Word3 Store sum BCR B'1111',14 Return to caller Word1 DC F'4' Fullword initially 4 Word2 DC F'6' Fullword initially 6 Word3 DS F Rsrvd only, no init End Add2 What macro processing does The program is "pre-processed" (plus a bit more advanced stuff) by the assembler For any unknown operation code, the assembler first searches previously defined programmer macros, and if that fails then it will search the macro library for a matching definition. What macro processing does When it finds such a definition, it processes it, usually generating assembler source statements from the model statements It is very important to understand that the eventual assembly will include the statements generated by macro expansions These can be seen in the program listing with a '+' to the left of the statement 13-16

5 What macro processing does Here we can see that the resulting object code will be exactly the same as in the Demo without the macro And object code is all that matters What macro processing does "Big deal," you may say - cut-and-paste can accomplish the same thing, right? Sure, when the macro is this simple But there's a lot more to come This was just to get acquainted with the ideas We will shortly be writing "programs" in the macro language, and these will select which assembler statements are generated, and how The Second Version of Our Add Macro In which we add a little complexity A new type of symbol Until now, every symbol has been a label with unchanging and unchangeable values (location, length, etc.) Examples: Data123, HEX, a9 We will call these ordinary symbols Another type of symbol can have a value assigned to it It is called a variable symbol Variable symbols begin with an ampersand & Examples: &X, &Data123, &NUM

6 A new type of symbol The first kind of variable symbol we will use is the symbolic parameter Later we will see the other two kinds: set symbols and system variable symbols Symbolic parameters appear in the operand field of the macro prototype statement When the macro is invoked, values specified in the operand field are assigned to the corresponding symbolic parameters A new type of symbol Symbolic parameters come in two types Positional (we will use these first) Keyword Now let's change our macro definition to have three symbolic parameters, each to be assigned the label of a storage area First will be the label of the first number Second will be the label of the second number Third will be the label of the storage area to hold the result Add Macro with Symbolic Parameters Macro Add &Num1,&Num2,&Result L 1,&Num1 Load 1st no. into R1 L 2,&Num2 Load 2nd no. into R2 AR 1,2 Get sum in R1 ST 1,&Result Store sum MEnd Source program before processing the macro instruction... Add2 CSect LR 12,15 Copy addr of 1st inst Using Add2,12 Tell assembler Add Word1,Word2,Word3 BCR B'1111',14 Return to caller Word1 DC F'4' Fullword initially 4 Word2 DC F'6' Fullword initially 6 Word3 DS F Rsrvd only, no init End Add

7 Source program after processing the macro instruction Add2 CSect LR 12,15 Copy addr of 1st inst Using Add2,12 Tell assembler Add Word1,Word2,Word3 + L 1,Word1 Load 1st no. into R1 + L 2,Word2 Load 2nd no. into R2 + AR 1,2 Get sum in R1 + ST 1,Word3 Store sum BCR B'1111',14 Return to caller Word1 DC F'4' Fullword initially 4 Word2 DC F'6' Fullword initially 6 Word3 DS F Rsrvd only, no init End Add2 Using symbolic parameters We can specify any operand values we wish: Add X,Y,Z Add Val,=F'1',Val The second invocation generates: + L 1,Val Load 1st no. into R1 + L 2,=F'1' Load 2nd no. into R2 + AR 1,2 Get sum in R1 + ST 1,Val Store sum which simply adds 1 to the value at Val (there are much better ways to do this!) Using symbolic parameters If we omit the third operand of Add we get Add Num1,Num2 <-omitted 3rd operand + L 1,Num1 Load 1st no. into R1 + L 2,Num2 Load 2nd no. into R2 + AR 1,2 Get sum in R1 + ST 1, <-error Store sum This error will not be detected during macro processing, but the assembly process will certainly object to a missing operand This demonstrates that the initial value of a symbolic parameter can be the null string Writing comment statements The usual assembler comment statement has an asterisk '*' in column 1 Assembler comments within a macro instruction are displayed as usual These comments are intended to document the generated assembler program Macro comments have '.*' in columns 1 and 2 and are visible only in the macro definition (they never appear in an expansion) These are used to document the macro itself 25-28

8 Labels on the prototype statement It may be useful to have a label on the first executable instruction, perhaps to allow a "branch" to the generated code This is accomplished by placing a variable symbol in the label field of the prototype, then also placing it in the label field of the first executable instruction generated Our next example demonstrates this, as well as both types of comment Add macro with comments Macro &Label Add &Num1,&Num2,&Result.* * * * * * * * * * * * * * * * * * * * * * * *.*.* This macro generates instructions to add two.* fixed point numbers at &Num1 and &Num2, and.* to save the result at &Result.*.* * * * * * * * * * * * * * * * * * * * * * * * * Add two numbers and save the result &Label L 1,&Num1 Load 1st no. into R1 L 2,&Num2 Load 2nd no. into R2 AR 1,2 Get sum in R1 ST 1,&Result Store sum MEnd Source program after processing the macro instruction Add2 CSect LR 12,15 Copy addr of 1st inst Using Add2,12 Tell assembler GoHere Add Word1,Word2,Word3 +* Add two numbers and save the result +GoHere L 1,Word1 Load 1st no. into R1 + L 2,Word2 Load 2nd no. into R2 + AR 1,2 Get sum in R1 + ST 1,Word3 Store sum BCR B'1111',14 Return to caller Word1 DC F'4' Fullword initially 4 Word2 DC F'6' Fullword initially 6 Word3 DS F Rsrvd only, no init End Add2 The Macro Programming Language In which we explore a high level language to 'create' assembler language programs 29-32

9 Macro language program structure Up to this point, the assembler code produced by our macro has been "unconditional" in that the same four model statements have been used This is comparable to a computer program in which the only program structure is a sequence without branches So, we need a "branch" capability, and this will give us conditional assembly Macro language program structure Remember: the purpose of a macro is to generate assembler language statements With "branch in assembly" statements we can add selection (if/then) or iteration (loop) to the assembly process But if we are to "branch," we need to label the destination This kind of label is a sequence symbol Sequence symbols and branching A sequence symbol looks like an ordinary symbol except it is preceeded by a '.' Examples:.Data123,.HERE There are two "branch in assembly" statements which reference sequence symbols (not completely defined here) AGo sequence_symbol AIf (logical_expression)sequence_symbol Sequence symbols and branching An example of the AGo instruction is AGo.there Of course, the label.there must appear somewhere inside the macro Before we can understand the AIf statement we have to understand what is meant by the "logical_expression" part of the operand First we look at arithmetic expressions 33-36

10 Arithmetic expressions (Keep in mind that we are discussing the pre-assembly phase, not assembly) An arithmetic expression is one formed from Self-defining terms (123, 09, etc.) The operators +, -, *, and / Variable symbols (beginning with &) Parentheses for grouping Arithmetic expressions Some examples of arithmetic expressions 5495 (25/4) &ctr+1 (&this*4)-&that &this*(4-&that) We will see much more of these after we encounter the second kind of variable symbol, the set symbol Logical expressions (simplified) A logical expression is one of the following A 0 or 1 Two arithmetic expressions, each enclosed in parentheses, separated by a relational operator (EQ, NE, LT, GT, LE, GE) A character string enclosed in apostrophes, and a symbolic parameter enclosed in apostrophes, separated by a relational operator and all enclosed in apostrophes In all cases, the expression is evaluated to either 0 or 1 The AIf instruction AIf is the conditional branch instruction seq_sym1 AIf (log_exp)seq_sym2 The first sequence symbol (seq_sym1) is in the label field and is optional If the value of the logical expression log_exp is 1, the next statement processed by the assembler is the one which has seq_sym2 in its label field If the value of log_exp is 0, the next statement processed by the assembler is the one after the AIf 37-40

11 The AGo and ANOP instructions AGo is the unconditional branch instruction seq_sym1 AGo seq_sym2 The first sequence symbol (seq_sym1) is in the label field and is optional The next statement processed by the assembler is the one which has seq_sym2 in its label field seq_sym ANOP Simply provides a place for a sequence symbol label (like DS 0H for ordinary symbols) Add macro with AIf and ANOP Macro &Label Add &Num1,&Num2,&Result.* * * * * * * * * * * * * * * * * * * * * * * *.* This version checks for a third parameter..* If it is missing, no instructions are.* generated,thus avoiding assembly errors..* * * * * * * * * * * * * * * * * * * * * * * * &Label DS 0H Label holder AIf ('&Result' EQ '').done L 1,&Num1 Load 1st no. into R1 L 2,&Num2 Load 2nd no. into R2 AR 1,2 Get sum in R1 ST 1,&Result Store sum.done ANOP MEnd Add macro with AIf and ANOP The goal is to not generate any instructions if the third operand is missing &Label has been moved to a DS 0H which is always generated, so any label specified on ADD will be present.done is on an ANOP as a demonstration, but could as easily have been on MEnd Macro messages The MNote instruction can provide: the programmer with an error or informational message within the expansion a candidate for the assembly return code seq_sym MNote [code,]'message' If code is an asterisk or is omitted along with the comma, this is an informational message If code is omitted and comma is present, a default code of 1 is used If code is present, it is the candidate 41-44

12 The MExit instruction seq_sym MExit Provides an exit for the assembler from any point within a macro definition The assembly is resumed with the next sequential instruction after the instruction that invoked the macro MNote and MExit will be demonstrated in our final Add example What we can expect tomorrow! This is similar to the result for Add2, except that it uses the RX add A instead of AR * Add (Word1,Word2),Word3 + L 1,Word1 Increment binary part of sum + A 1,Word2 Increment binary part of sum + ST 1,Word3 Save result... * Word1 DC F'4' Word2 DC F'6' Word3 DS F * What we can expect tomorrow! AddDemo Add (One,Two,Three,Two,Three,One),Three +AddDemo L 1,One Increment binary part of sum + AH 1,Two Increment binary part of sum + ZAP DWrd0009,Three Increment decimal part of sum + AH 1,Two Increment binary part of sum + AP DWrd0009,Three Increment decimal part of sum + A 1,One Increment binary part of sum + CVB 0,DWrd0009 Get binary sum of dec summands + AR 1,0 Add to sum of binary summands + CVD 1,DWrd0009 Get decimal sum of summands + ZAP Three,DWrd0009 Copy to target + B DWrd Branch around doubleword +DWrd0009 DS D Scratch area... * One DC F'1' Two DC H'2' Three DC PL2'3' 45-48