Assembler Programming. Lecture 6

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1 Assembler Programming Lecture 6

2 Lecture 6 Full segment definitions. Procedures. Parameters. Modules.

3 Full segment definitions Complete control over segments. Control over segments order. Posibility of groupping segments.

4 SEGMENT directive name SEGMENT [[align]] [[READONLY]] [[combine]] [[use]] [[class]] Name the name of the segment. Align memory boundary of beginning. READONLY generate an error while modifying item in the segment. Combine how to combine segments. Use size of the segment (16 or 32 bit). Class segments of the same class are groupped together.

5 SEGMENT directive name SEGMENT [[align]] [[READONLY]] [[combine]] [[use]] [[class]] BYTE - Next available byte address. WORD - Next available word address. DWORD - Next available doubleword address. PARA - Next available paragraph address (16 bytes per paragraph). Default. PAGE - Next available page address (256 bytes per page).

6 SEGMENT directive name SEGMENT [[align]] [[READONLY]] [[combine]] [[use]] [[class]] Protects against illegal sefl-modyfying code. Assembler checks for the instructions that modify the segment. Generates an error if You attempt to write directly to read-only segment.

7 PRIVATE - Does not combine the segment with other segments, even if they have the same name. Default. PUBLIC - Concatenates all segments having the same name to form a single, contiguous segment. STACK - Concatenates all segments having the same name and set SS:SP to the top of the segment. COMMON - Overlaps segments. The length is the length of the largest of the combined segments. MEMORY - Used as a synonym for the PUBLIC. AT address - Assumes address as the segment location. SEGMENT directive name SEGMENT [[align]] [[READONLY]] [[combine]] [[use]] [[class]]

8 SEGMENT directive name SEGMENT [[align]] [[READONLY]] [[combine]] [[use]] [[class]] USE16 offset is 16-bit. USE32 offset is 32-bit. FLAT.

9 SEGMENT directive name SEGMENT [[align]] [[READONLY]] [[combine]] [[use]] [[class]] Two segments of the same name are not combined if their class is different. All segments with the same class are next to each other.

10 Controlling segment order.seq order in which segments are declared..alpha alphabetical order within a module..dosseq: Code segments, Data segments: Segments not in class BSS or STACK, BSS segments, STACK segments.

11 Defining segment group name GROUP Segment [[, segment ]] Collection of segments not greater than 64kB. Group example is DGROUP containing _DATA, _BSS, CONST and STACK segments. Name is the label of the group. Segment can be any segment name. Segment can t belong to more than one group. Does not affect segments order 16-bit and 32-bit segments can t be in the same group.

12 Groupping example MYGROUP GROUP ASEG, BSEG ; MYGROUP contains two two ; segments ASEG and and BSEG MYGROUP GROUP CSEG ; Adds the the CSEG segment into ; MYGROUP so so it it contains now now ; three segments ASSUME DS:MYGROUP ; DS DS refers to to MYGROUP mov mov AX,MYGROUP mov mov DS,AX

13 Assuming segment registers Many instructions work only on offset part of the adress. They assume they work in default segment. Assembler must know what segment contains the address. ASSUME directive.

14 ASSUME directive ASSUME segregister : seglocation Segregister is the name of the segment register. Seglocation must be the name of the segment or the group. CS is automatically assumed with code segment. ASSUME affects only assembler not the processor, You should set values of segment registers with instructions.

15 ASSUME directive ASSUME ASSUME segregister : seglocation [,segregister : seglocation] ASSUME ASSUME datareg : qualifiedtype [, [, datareg : qualifiedtype] ASSUME ASSUME register : ERROR ERROR [, [, register : ERROR] ERROR] ASSUME ASSUME [register :] :] NOTHING [, [, register : NOTHING] ASSUME ASSUME register : FLAT FLAT [, [, register : FLAT] FLAT] NOTHING cancels the assumptions. ERROR prevents the use of register.

16 Full definitions example Staseg StasegSEGMENT STACK STACK Staseg Staseg ENDS ENDS Dtaseg Dtaseg SEGMENT ; Place Place variables here here Dtaseg Dtaseg ENDS ENDS Codseg Codseg SEGMENT ASSUME ASSUME DS:Dtaseg, SS:Staseg mov mov AX,Dtaseg mov mov DS,AX DS,AX ; Place Place program here here Codseg Codseg ENDS ENDS END END

17 Procedures Divide large programs into manageable units. Allow separate testing. Make code more efficient for repetitive tasks. Make easier using code in future applications.

18 Procedure definition PROC directive starts the procedure. ENDP directive ends the procedure. Additionally, PROC can automatically: Preserve register values that should not change but that the procedure might otherwise alter. Set up a local stack pointer, so that you can access parameters and local variables placed on the stack. Adjust the stack when the procedure ends.

19 Procedure definition example My_proc PROC FAR ret My_proc ENDP RET [constant] instruction.

20 Procedure calling CALL instruction Pushes the address of the next instruction on the stack. Passes control to a specified address. Address can be label, register or memory. Operand is calculated at run time. Calls can be near or far.

21 Procedure definition Procedures can be nested My_proc PROC NEAR... Your_prc PROC NEAR... ret Your_prc ENDP... ret My_proc ENDP

22 Procedure definition without PROC and ENDP NEAR procedures: Call NEAR PTR My_proc Call NEAR PTR Your_prc My_proc: retn Your_prc LABEL NEAR retn

23 Procedure definition without PROC and ENDP FAR procedures: Call FAR PTR My_proc My_proc LABEL FAR retf

24 Parameters of the procedure Parameters can be passed to the procedure: Through the registers. Through the common memory (variables). Through the stack. Mixed. Extended PROC syntax allows passing parameters automatically.

25 Extended PROC syntax label PROC [[attributes]] [[USES reglist]] [[Parameter[[:tag]]...]] Automates accessing arguments and saving registers. Attributes distance, langtype and visibility, Reglist list of registers that should be saved, Parameter list of parameters passed to the procedure on the stack. Label is the name of the procedure PROC is a required keyword

26 Extended PROC syntax label PROC [[attributes]] [[USES reglist]] [[Parameter[[:tag]]...]] [[distance]] [[langtype]] [[visibility]] [[<prologuearg>]] Distance NEAR or FAR, for 386 also NEAR16, NEAR32, FAR16, FAR32. Langtype BASIC, FORTRAN, PASCAL, C, STDCALL or SYSCALL.

27 Extended PROC syntax label PROC [[attributes]] [[USES reglist]] [[Parameter[[:tag]]...]] [[distance]] [[langtype]] [[visibility]] [[<prologuearg>]] Visibility PRIVATE, PUBLIC, EXPORT. Prologuearg LOADDS, FORCEFRAME.

28 Extended PROC syntax label PROC [[attributes]] [[USES reglist]] [[Parameter[[:tag]]...]] Reglist: Must be separated by spaces or tabs Assembler generates prologue code to push registers on the stack. Assembler generates epilogue code to pop the saved registers off the stack.

29 Extended PROC syntax label PROC [[attributes]] [[USES reglist]] [[Parameter[[:tag]]...]] List of parameters. Parameter - parmname [[:tag]] Parmname is the name of the parameter. Tag is the qualifier of type of the parameter. Tag can be the VARARG keyword for procedures with variable number of parameters.

30 Procedure parameters example My_add PROC NEAR C, arg1:word, arg2:word, count:word mov ax, arg1 add ax, count add ax, arg2 ret My_add ENDP

31 LOCAL directive. Local variables Creates local variables on the stack. Must be on the line following the PROC statement. Syntax of LOCAL directive: LOCAL vardef [[, vardef]]... Each vardef represents one local variable: Label [[ [count] ]] [[:type]]

32 Local variable example My_proc PROC NEAR arg:word LOCAL loc:word.. mov loc, 3 ; Initialize local variable add ax, loc ; Add local variable to to AX AX sub arg, ax ax ; Subtract local from arg.. ret My_proc ENDP

33 Procedure prototypes PROTO directive. The same function as in other HLL. Includes name, types and names of parameters. Enable the assembler to check for unmatched parameters. Useful for procedures called from other modules and other languages.

34 INVOKE directive Automatically pushes arguments onto the stack. Procedure must be declared previously. Generates a sequence of instructions that: Converts arguments to the expected types. Pushes arguments on the stack in the correct order. Calls the procedure. Cleans the stack when the procedure returns.

35 Prototype and invoking example. My_proc PROTO NEAR, C argcount:word, arg1:word, arg2:word My_proc PROC NEAR, C argcount:word, arg2:word, arg3:word... My_proc ENDP INVOKE My_proc, ax, x, x, y

36 Program modules Big projects can be divided into modules. Modules can be libraries for other languages. Symbols that have to be used in other modules must be visible in theese modules. Twomethods: With include file. Without include file.

37 Include files Can contain any MASM statement. If used in more than one module can t contain statements that define and allocate memory for symbols. INCLUDE directive: INCLUDE filename Filename must be fully specified (with the extension).

38 Modules with include file EXTERNDEF directive Public in the defining module External in referencing modules For variables and procedures COMM (communal) As EXTERNDEF but are allocated by linker Can t be initialized For variables PROTO For procedures Automatically issues EXTERNDEF

39 Include file example Include prcmod.inc My_proc PROTO NEAR, C argcount:word, arg1:word, arg2:word My_proc PROC NEAR, C argcount:word, arg2:word, arg3:word My_proc ENDP Include prcmod.inc My_proc PROTO NEAR, C argcount:word, arg1:word, arg2:word INVOKE My_proc, ax, ax, x, x, y

40 PUBLIC Modules without include file Makes a name visible outside the module in which it is defined. Must appear in defining module. EXTERN Referenced name is defined and declared public in another module. PUBLIC and EXTERN work with variables and procedures

41 PUBLIC and EXTERN example.model small, Pascal PUBLIC Build_table, Var Var LABEL FAR FAR DATA Var Var BYTE CODE Build_table PROC USES cx, cx, dx, dx, sizevar:word ret ret Build_table ENDP EXTERN Var:BYTE, Build_table:FAR mov mov al, al, Var Var call Build_table......

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