SOFTWARE ENVIRONMENT

Transcription

1 Chapter 5 SOFTWARE ENVIRONMENT As stated in chapter 2, a computer software is the collection of programs that can be executed on that computer. Programs are usually subdivided into two major classes: application programs and system programs. An application program is a program that is used to perform a task or to solve a problem for its user whereas a system program is a program that is used for the operation of the computer. Examples of application programs are word processing software such as Microsoft Word, electronic spreadsheets software such as Lotus 123, and data base management systems such as Microsoft Access. This chapter discusses the operating system used on Intel 8086based computer systems, and the system programs that are essential for the development and execution of programs. 5.1 System Programs Examples of system programs are text editors, compilers, assemblers, linkers, loaders, and operating systems. The operating system is the most fundamental of all system programs and text editors, compilers, assemblers, linkers, and loaders are essentials for the development and execution of programs. 119

2 120 Text Editors A text editor reads characters from the keyboard and places them into a file. Text editors are used to produce the source modules (or files) of a program. Most compiler and assembler software packages come with their own text editor. However, one called Edit is provided in the DOS environment of Windows operating systems, and another one called notepad comes with Windows operating systems. Compilers A compiler translates a highlevel language program source module into machine language and also creates an object file (module) that contains the machine language code translation of the program. There is a compiler for each language and for each type of computers. For examples, a C++ compiler for the IBM PC and compatible computers, a C++ compiler for the Apple Macintosh computers, or a Java compiler for the IBM PC and compatible computers. Assemblers An assembler translates an assembly language program source module into machine language and also creates an object file (module) that contains the machine language code translation of the program. In addition to containing the machine language translation of a source module, an object module also contains other information such as references to external data and entry points of external procedures and functions that must be resolved by the linker. It also contains relocatable values that may change after an object module is combined with other object modules. A relocatable value is a value such as the displacement of a memory location that depends on the starting address of the program data or stack segment. It also contains references to addresses that are not known until the program is loaded in memory. One such reference is a segment selector in Intel 8086 systems. Part of this information is processed by the linker, and the rest is processed by the loader.

3 121 Linker A linkage editor, or linker combines separately translated object modules and the library functions called in a program into a single load (or executable) module. It must perform two major tasks: the first is to concatenate all the object codes into one module, possibly adjusting the relocatable values (such as displacements) that are to be relative to the start of the final module for object files other the first one. In an Intel 8086based system, this process involves the concatenation of related segments from all the object files and the libraries. The second task is to resolve external references in each object module by providing the address or the pointer to each external reference in an object module. In addition to the instructions and the data of a program, the load module also contains the program header which is used to provide information about the load module to the loader. Some of the information provided in the program header are the size of the load module, the size of the program header, and information about references to addresses that are not known until the program is loaded in memory. Loaders Using the information provided in the program header, A loader does the following: determine how much memory is needed to hold a program, and the starting address for a program. copy the load module into memory. For the Intel 8086based systems, the loader also builds a program segment prefix (PSP) in the first 256 (100h) bytes of the memory allocated to a program. The PSP is used by the operating system DOS to save certain information about the system when the program is invoked. It is also used to hold the command line arguments of the program. adjust references to addresses that depend on the starting address of the program by adding a constant to them.

4 122 initialize the registers that are necessary for the execution of the program. For example, in computer systems based on the Intel 8086 processor, registers SP and IP are initialized with values provided in the program header, and the segment registers CS and SS are initialized respectively with the selector of the code segment and the selector of the stack segment. transfer the control of the CPU to the program for execution. In Intel 8086based systems, the transfer is accomplished by setting the initial values of the registers CS and IP. Operating Systems The operating system is the most fundamental of all system programs. It manages the system resources, main memory, disks, printers, CPU,.., etc, and also provides an interface to the hardware that is easy to understand and use in other programs. The operating systems used on most Intel Pentium systems today are windows 98, and windows However, windows 2000 provides a program called NTVDM (NT virtual DOS machine) that emulates the operating system DOS and the ROM BIOS which constitute the major operating system used on computer systems based on the Intel 8086 processor. 5.2 DOS and ROM BIOS The operating system used on most Intel 8086based computer systems consists of DOS (disk operating system) and the ROM BIOS (basic input/output system). The ROM BIOS are routines that are for the most part machine independent. They are stored in read only memory on a chip that plugs on the motherboard. These routines consist of computer initialization routines, and service routines that may be used by application programs and DOS to have access to the hardware.

5 123 MSDOS (PC DOS for IBM PC) developed by Microsoft Corporation consists of three files: two hidden files (that do not show up in the disk directory) called IO.SYS and MSDOS.SYS (IBMBIO.SYS and IBMDOS.COM on IBM PC) and another file, the command interpreter called COMMAND.COM. IOS.YS (IBMBIO.SYS on IBM PC) usually called DOS BIOS has three elements: resident device drivers customized for each particular machine by the manufacturer; a BIOS initialization routine, and the system initialization routine called SYSINT, which is provided by Microsoft. DOS BIOS must contain device drivers for at least the following devices: CON (keyboard and display), PRN (line printer LPT1), AUX (serial port COM1), CLOCK$ (date and time support), and one disk drive (block device). MSDOS.SYS (IBMDOS.COM on IBM PC) also called the kernel of DOS is developed and owned by Microsoft Corporation. It provides service routines for file and directory management, character device input and output (keyboard, screen, and printer), and time and date support. It also provides service routines for memory management and task and environment management. These services call upon BIOS services and device drivers to provide a device independent access to the hardware that is easy to understand and use in application programs. The CONFIG.SYS file that also comes with DOS allows certain modifications to the kernel and the addition of other device drivers to the BIOS. The command interpreter (also called DOS shell) COMMAND.COM provides a commandoriented interface between the user and the system. It contains routines that may be invoked through (internal) commands issued by a user to perform operations such as file and directory operations, and operations to load and execute programs. It displays a prompt called DOS prompt, monitors the keyboard, and processes user commands. You open a window to run the command interpreter by doing the following: 1. Click on START 2. Follow the path: All Programs > Accessories > Command Prompt 3. Click on Command Prompt. (A prompt like the following one will be displayed: C:\>) You exit the DOS command interpreter and return to Windows by typing the command EXIT at the DOS Prompt.

6 124 DOS Program Files DOS recognizes two types of executable or program files: files with filename extension.com (.COM files), and those with filename extension.exe (.EXE files). The two major differences between these program files follow: S S COM program cannot exceed 64 KB in size. In a.com program, all the (logical) segments are loaded in the same frame. That means, the contents of all segment registers are set to the same segment selector. An.EXE program is produced by the linker from the object file(s) of a program. However, some.exe programs may be converted into a.com program by using EXE2BIN utility program. Both kinds of programs (.COM and.exe) are executed just by typing their names followed by a carriage return at the DOS prompt. DOS also recognizes another type of files (with extension.bat) called command file. A command file is used by computer users to hold the sequence of DOS commands that they would otherwise provide to DOS from the keyboard. InputOutput Interface DOS also includes a number of routines called service routines or system services which can be invoked in userdefined (or application) programs to access the hardware. Some of these service routines are used for program termination and return to the command processor or for accessing peripheral devices (for example to perform I/O operations). Each service routine (or function) is identified by an 8bit (00h to FFh) code called function code, and is accessed by application programs through interrupts. BIOS (Basic Input/Output System) which is burned into the ROM of 8086based computers by the manufacturer also provides service routines to be used by DOS and application programs to access the hardware. BIOS services are more efficient and allow the programmer and DOS to get closer to the hardware. DOS services and application programs use interrupts to access BIOS services.

7 125 A program initiates an interrupt by using the INTerrupt instruction with the following syntax: Examples INT <type> Assembly Language Machine Language INT 3 INT 0Ah CC CD 0A INT 5 CD 05 INT 20h is used to terminate a.com program and to return the control of the CPU to the command processor. DOS service routines are accessed by loading their function code into register AH and issuing the INT 21h (also called DOS function call) instruction. 5.3 Using Debug The DOS operating system comes with a utility program named DEBUG.COM which is used for testing and debugging executable (.COM and.exe) programs. In the Debug environment, the programmer can store the instructions and the data of a machine language program into the memory and execute it. Debug also has a singlestep execution mode that allows a programmer to execute a program one instruction at a time, so that he/she can view the effects of each instruction on memory locations and the registers before the execution of the next instruction. You start Debug by typing one of the following commands at the DOS prompt: DEBUG or DEBUG <filespecification> where <filespecification> is the filename specification of an executable (.COM or.exe) program file.

8 126 If <filespecification> is specified, Debug immediately loads the specified file into the memory for debugging, and initializes the registers CS, IP, SS, and SP. Upon entry, Debug displays a dash () to prompt the user for a command. Debug Commands A Debug command consists of a single character. Some commands may also have one or more parameters which may either be a (physical) address, a range of addresses, a register name, or a value. All values are specified in hexadecimal (with up to four digits) without the h or H suffix. A word value with less than four digits is padded to the left with zeroes. An address may be specified in one of the following forms: Examples: selector:offset or Segregister:offset S S S 1BF4:005A CS:005A DS:0008 When a segment register is used, Debug assumes that the segment selector part of the address is the contents of that register. The selector or Segregister may also be omitted as in : 005A and 008. When this is the case, Debug either uses the contents of register DS (data segment) or the contents of register CS (code segment) as the selector depending on the command. A range of addresses is used to specify the first and the last byte of a continuous block of bytes in the main memory. It is specified in one of the following forms: <address1> <offset2> or <address1> L n

9 127 where <address1> <offset2> n is the selector:offset address of the first byte, is the offset of the last byte is the number of bytes (in hexadecimal) in the block. Examples: S 1BF4: memory location from offset 0120 to offset 0140 in the segment with selector 1BF4. S CS:350 37F Memory location from offset 0350 to offset 037F in the segment with selector in register CS (code segment). S 1BF4:120 L 21 Memory location with 21h (= 33) bytes starting at offset 0120 in the segment with selector 1BF4. Same memory location as the one in the first example. S CS:350 L 30 Memory location with 30h ( = 48 ) bytes starting at offset 0350 in the segment with selector in CS (code segment). Same memory location as the one in second example. Debug does not distinguish between lowercase and uppercase letters and each command must be terminated with the ENTER key. The following are the most commonly used Debug commands: R Display the contents of all registers. The contents of each register are displayed in hexadecimal. On the line following the contents of the registers, it displays the next instruction to be executed (in symbolic form) with its address. Example R AX=0000 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=0100 NV UP EI PL NZ NA PO NC 0AE7: CBW

10 128 R <registername> change the contents of register with name <registername>. Debug first displays the old contents of the specified register (in hexadecimal). To change it, type the new value (in hexadecimal) then press ENTER. Just press ENTER to retain the old value. Example The contents of register AX is changed to 1A1B as follows: R AX AX 0000 (Current contents of register AX) :1A2B R AX AX 1A2B (New contents of register AX) D [ range ] Display (dump) the contents of a memory location specified by <range>. The contents of the memory location are displayed in hexadecimal (with a maximum of16 bytes per line). with the ASCII character representation of each byte (if any) to the far right. The format of a displayed line is as follows: <Selector: Offset Address> < Hexadecimal Representation> <ASCII characters> Examples: 1. To display the contents of the memory location from offset 0120 to offset 013F in the data segment: D DS:120 13F 0AE7:0120 A A C9 75 1D 0AC B 0E E3.$...u...t...!.. 0AE7: B0 1A FF 8E F2 AE F u.O 2. To display 14 bytes from offset 110 in the data segment: D 110 LE 0AE7: D0 E0 D0 E0 A2 1E E > 4.

11 129 Note the following: The default segment for the D command is the data segment. If a byte does not correspond to a valid ASCII code, the corresponding character is a dot (.) If a range is not specified in a D command, 128 bytes starting at the next offset after the last display (or DS:100 for the first display) will be displayed. In the case that only the beginning offset is specified as in the command: D 12A 128 bytes will be displayed starting at that address. E <address> <valuelist> Enter one or more byte values specified by <valuelist> into the memory location starting at <address>. <valuelist> is a sequence of one or more byte values and/or string of characters separated by spaces. A character string must be enclosed inside quotes. Characters in a string are translated into their ASCII codes. Examples: 1. Store the bytes values 0A, 1B, 2C, 3D, and 4F into memory starting at the address DS:110: E DS:110 A 1B 2C 3D 4E 5F D DS:110 L6 0AE7:0110 0A 1B 2C 3D 4E 5F..,=N_ 2. Store the byte values 0A and 0B, and the string CS280' into memory starting at the address DS:120: E 120 A B 'CS280' D 120 L7 0AE7:0120 0A 0B CS280

12 Store the string Gilbert Ndjatou into memory starting at offset 0130: E 130 'Gilbert Ndjatou' D 130 LF 0AE7: C E 64 6A F 75 Gilbert Ndjatou Note that if <list> is omitted, Debug displays the address and its current contents. To change it, type the new value, then press ENTER; to leave the byte unchanged, press the SpaceBar if you want to display the contents of the next address; otherwise, press ENTER. A <address> Translate (assemble) Assembly Language Instructions into machine language and store them into consecutive memory locations starting at <address>. Type each instruction followed by ENTER; and press ENTER if there is no more instruction. If <address> is not specified, the first machine language instruction is stored at the address specified by CS:IP. The default segment for this command is the code segment. Example: A 100 0AE7:0100 MOV AX, 1 0AE7:0103 MOV BX, 2 0AE7:0106 MOV CX, 3 0AE7:0109 MOV DX, 4 0AE7:010C INT 20 0AE7:010E

13 131 U <range> Translate (disassemble) the machine language instructions in the memory locations specified by <range> to assembly language. If the range is not specified, the translation starts at the first address after the last translation (or CS:IP for the first time). If only the first address is specified, instructions in approximately twenty bytes are translated. Examples U D 0AE7:0100 B80100 MOV AX,0001 0AE7:0103 BB0200 MOV BX,0002 0AE7:0106 B90300 MOV CX,0003 0AE7:0109 BA0400 MOV DX,0004 0AE7:010C CD20 INT 20 T < =address> <value> Executing (trace) the Machine Language Instructions in memory. <address> is the address of the first instruction to be executed and <value> (in hexadecimal) is the number of instructions to be executed. After the execution of each instruction, Debug displays the contents of all registers, and the next instruction to be executed. If <address> is not specified, the execution begins with the instruction at address CS:IP. If <value> is not specified, only one instructions is executed. Just type the command T again to execute the next one. Examples: 1. Execute 4 instructions starting with the instruction at offset 0100: T=100 4 AX=0001 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=0103 NV UP EI PL NZ NA PO NC 0AE7:0103 BB0200 MOV BX,0002 AX=0001 BX=0002 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=0106 NV UP EI PL NZ NA PO NC

14 132 0AE7:0106 B90300 MOV CX,0003 AX=0001 BX=0002 CX=0003 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=0109 NV UP EI PL NZ NA PO NC 0AE7:0109 BA0400 MOV DX,0004 AX=0001 BX=0002 CX=0003 DX=0004 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=010C NV UP EI PL NZ NA PO NC 0AE7:010C 06 PUSH ES 2. Execute one instruction (at a time) starting with the instruction at offset 0100: T=100 AX=0001 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=0103 NV UP EI PL NZ NA PO NC 0AE7:0103 BB0200 MOV BX,0002 T AX=0001 BX=0002 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0AE7 ES=0AE7 SS=0AE7 CS=0AE7 IP=0106 NV UP EI PL NZ NA PO NC 0AE7:0106 B90300 MOV CX, G <=address> <address1> <address2>... Execute machine language instructions in memory starting at <address>. <address1>, <address2>,... are called break points. A break point must be the address of the first byte of an instruction in the RAM. (Break points cannot be set in the ROM). Execution of the program stops at each break point and the control of the CPU is passed back to DEBUG. The contents of all registers are also displayed. Up to ten break points may be specified. If <=address> is not specified, the execution begins with the instruction at CS:IP. If there is no break point, the execution continues until an instruction that terminates the execution of the program is executed.

15 133 Examples: G =100 10A 110 Execute instructions starting at offset 0100 and stop at instruction at offset 010A G Execute instructions starting at offset 010A and stop at instruction at offset 0110 G Execute instructions starting at offset 0110 N <FileSpecification> Define (name) the file to be used in any subsequent LOAD/WRITE file command. W <address> Write (BX CX) number of bytes from memory starting at <address> to the file previously defined with the Name command. This command cannot be used to write.exe files. It is mostly used to save Debug programs into.com files. The length of the program in bytes must first be placed as a doubleword in the registers pair (BX CX). If <address> is not specified, the beginning address will be CS:100. Example: A 100 0AE7:0100 MOV AX, 1 0AE7:0103 MOV BX, 2 0AE7:0106 MOV CX, 3 0AE7:0109 MOV DX, 4 0AE7:010C INT 20 0AE7:010E N lab.com R BX BX 0000 :0 R CX CX 0000 :E W 100 Writing 0000E bytes (These bytes are written into the file LAB.COM).

16 134 L <address> Load a file from disk into the memory starting at <address>. The file must have been previously defined using the Name command. If <address> is not specified, nonexecutable files are loaded starting at address DS: Debug examines executable (.EXE and.com) files to determine how to load them. After a file has been loaded, Debug set the registers pair (BX CX) to a doubleword that corresponds to the number of bytes loaded from the file. Example: The program saved above in the file LAB.COM is reloaded in the memory starting at offset 0100: N LAB.COM L CS:100 R AX=0000 BX=0000 CX=000E DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000 DS=0B2E ES=0B2E SS=0B2E CS=0B2E IP=0100 NV UP EI PL NZ NA PO NC 0B2E:0100 B80100 MOV AX,0001 U 100 LE 0B2E:0100 B80100 MOV AX,0001 0B2E:0103 BB0200 MOV BX,0002 0B2E:0106 B90300 MOV CX,0003 0B2E:0109 BA0400 MOV DX,0004 0B2E:010C CD20 INT 20 Q Quit Debug and return to the DOS prompt.