Comprehensive listing of interrupts and hardware details.
Parameters are passed to and from the BIOS routines using the 8088/86 CPU registers. The routines normally preserve all registers except AX and the flags. Some registers are altered if they return values to the calling process.
ROM BIOS INTERRUPT ROUTINES 10 Video routines 11 Equipment Check 12 Memory Size Determination 13 Diskette routines 14 Communications routines 15 Cassette 16 Keyboard routines 17 Printer 18 Cassette BASIC 19 Bootstrap loader 1A Time of DayThe interrupts which handle devices are like a gateway which provide access to more than one routine. The routine executed will depend upon the contents of a particular CPU register. Each of the software interrupt calls use the 8088/86 register contents to determine the desired function call. It is necessary to use a C definition of the CPU programming model, this allows the registers to be initialised with the correct values before the interrupt is generated. The definition also provides a convienent place to store the returned register values. Luckily, the definition has already been created, and resides in the header file dos.h. It is a union of type REGS, which has two parts, each structures.
One structure contains the eight bit registers (accessed by .h.), whilst the other structure contains the 16 bit registers (accessed by .x.) To generate the desired interrupt, a special function call has been provided. This function accepts the interrupt number, and pointers to the programming model union for the entry and return register values. The following program demonstrates the use of these concepts to set the display mode to 40x25 color.
#include <dos.h>
union REGS regs;
main()
{
regs.h.ah = 0;
regs.h.al = 1;
int86( 0x10, ®s, ®s );
printf("Fourty by Twenty-Five color mode.");
}
SCREEN DISPLAY MODE FUNCTION CALL (regs.h.ah = 0) regs.h.al Screen Mode 0 40.25 BW 1 40.25 CO 2 80.25 BW 3 80.25 CO 4 320.200 CO 5 320.200 BW 6 640.200 BW 7 Mono-chrome 8 160.200 16col PCjr 9 320.200 16col PCjr A 640.200 4col PCjr D 320.200 16col EGA E 640.200 16col EGA F 640.350 mono EGA 10 640.350 16col EGANote: The change screen mode function call also has the effect of clearing the video screen! The other routines associated with the video software interrupt 0x10 are, (Bold represents return values)
Function Call regs.h.ah Entry/Exit Values Set display mode 0 Video mode in al Set cursor type 1 Start line=ch, end line=cl (Block cursor, cx = 020C) Set cursor position 2 Row,column in dh,dl, page number in bh Read cursor position 3 Page number in bh, dh,dl on exit has row,column Read light pen pos 4 ah=0 light pen not active ah=1 light pen activated dh,dl has row,column ch has raster line (0-199) bx has pixel column (0-139,639) Select active page 5 New page number in al Scroll active page up 6 Lines to scroll in al(0=all) upper left row,column in ch,cl lower right row,col in dh,dl attribute for blank line in bh Scroll active page dn 7 Same as for scroll up Read char and attr 8 Active page in bh al has character ah has attribute Write char and attr 9 Active page in bh number of characters in cx character in al attribute in bl Write character 0a Active page in bh number of characters in cx character in al Set color palette 0b Palette color set in bh (graphics) color value in bl Write dot 0c Row,column in dx,cx color of pixel in al Read dot 0d Row,column in dx,cx color in al Write teletype 0e Character in al, active page in bh foregrnd color(graphics) in bl Return video state 0f Current video mode in al columns in ah,active page in bh
/* #include <conio.h> */ outp( Port_Address, value); /* turboC uses outportb() */ value = inp( Port_address); /* and inportb() */The various devices, and their port values, are shown below,
Port Range Device 00 - 0f DMA Chip 8737 20 - 21 8259 PIC 40 - 43 Timer Chip 8253 60 - 63 PPI 8255 (cassette, sound) 80 - 83 DMA Page registers 200 - 20f Game I/O Adapter 278 - 27f Reserved 2f8 - 2ff COM2 378 - 37f Parallel Printer 3b0 - 3bf Monochrome Display 3d0 - 3df Color Display 3f0 - 3f7 Diskette 3f8 - 3ff COM1
Port Value Register Description 3d0 6845 registers 3d1 6845 registers 3d8 DO Register (Mode control) 3d9 DO Register (Color Select) 3da DI Register (Status) 3db Clear light pen latch 3dc Preset light pen latch
Value Color Value Color 0 Black 8 Dark Grey 1 Blue 9 Light Blue 2 Green 0a Light Green 3 Cyan 0b Light Cyan 4 Red 0c Light Red 5 Magenta 0d Light Magenta 6 Brown 0e Yellow 7 Light Grey 0f WhiteThe following program will set the border color to blue.
#include <conio.h> /* needed for outp() */
#include <stdio.h> /* needed for getchar() */
#include <dos.h> /* for REGS definition */
#define CSReg 0x3d9
#define BLUE 1
void cls()
{
union REGS regs;
regs.h.ah = 15; int86( 0x10, ®s, ®s );
regs.h.ah = 0; int86( 0x10, ®s, ®s );
}
main()
{
cls();
printf("Press any key to set border color to blue.\n");
getchar();
outp( CSReg, BLUE );
}
38,28,2d,0a,1f,6,19,1c,2,7,6,7,0,0,0,0The default settings for the registers for the various screen modes can be found in ROM BIOS listing's and technical reference manuals.
To program the various registers, first write to the address register at port 3d4, telling it which register you are programming, then write the register value to port 3d5. The mode select register must also be programmed. This is located at port 3d8. The sequence of events is,
1: Disable the video output signal
2: Program each register
3: Enable video output, setting mode register
The following program illustrates how to do this,
#include <conio.h> /* needed for outp() */
#include <stdio.h> /* needed for getchar() */
#include <process.h> /* needed for system calls */
#define MODE_REG 0x3d8
#define VID_DISABLE 0
#define FOURTY_25 0X28
#define ADDRESS_REG 0x3d4
#define REGISTER_PORT 0x3d5
static int mode40x25[] = {0x38,0x28,0x2d,0x0a,0x1f,6,0x19,0x1c,
2,7,6,7,0,0,0,0 };
void cls()
{
union REGS regs;
regs.h.ah = 15; int86( 0x10, ®s, ®s );
regs.h.ah = 0; int86( 0x10, ®s, ®s );
}
main()
{
int loop_count = 0;
cls();
printf("Press a key to set 40x25 color mode.\n");
getchar();
outp(MODE_REG,VID_DISABLE); /*disable video signal */
for( ; loop_count < 0x0e; ++loop_count) {
outp( ADDRESS_REG, loop_count ); /* set up CRT register */
outp( REGISTER_PORT, mode40x25[loop_count]); /* write to reg */
}
outp( MODE_REG, FOURTY_25); /* switch in mode now */;
printf("Press key to exit.\n"); getchar();
}
The program should update the video_mode byte stored at 0040:0049
to indicate the change in video state. This type of low level
programming is an example of code required for embedded applications
(ie, ROM CODE running without DOS or the ROM BIOS chip present).
int86( interrupt_number, ®s1, ®s2); where int86() is the function call, interrupt_number is the interrupt to generate ®s1 defines the input register values ®s2 defines the returned register valuesInt 10h is the interrupt to use for video calls. We will be using the set mode routine, so the values are,
regs.h.ah = 0; /* set mode function call */ regs.h.al = 1; /* set mode to 40x25 color */The following program illustrates how this all fits together,
#include <dos.h>
#include <stdio.h>
#include <process.h>
union REGS regs;
void cls()
{
regs.h.ah = 15; int86( 0x10, ®s, ®s );
regs.h.ah = 0; int86( 0x10, ®s, ®s );
}
main()
{
cls();
printf("Setting mode to 40x25 color.\n");
printf("Press any key....\n");
getchar();
regs.h.ah = 0;
regs.h.al = 1;
int86( 0x10, ®s, ®s);
printf("Mode has been set.\n");
}
Calls to the ROM BIOS Int 10h Set Video Mode routine also update
the video_mode flag stored at 0040:0049.
AH = 10h Select Colors in EGA/VGA AL = 1 BL = color register (0 - 15), BH = color to set AL = 2 ES:DX ptr to change all 16 colors and overscan number AL = 3 BL = color intensity bit. 0 = intensity, 1 = blinking (VGA systems only) AL = 7 BL = color register to get into BH AL = 8 BH = returned overscan value AL = 9 ES:DX = address to store all 16 colors + overscan number AL = 10h BX = color register to set; ch/cl/dl = green/blue/red AL = 12h ES:DX = pointer to change color registers, BX = 1st register to set, CX = number of registers involved AL = 13h BL = 0, set color page mode in BH BL = 1, set page number specified by BH AL = 15h BX = color register to read; ch/cl/dl = grn/blue/red AL = 17h ES:DX = pointer where to load color registers BX = 1st register to set, CX = number of registers involved AL = 1Ah get color page information; BL = mode, BH = page number AH = 11h Reset Mode with New Character Set AL = 0 Load new character set; ES:BP pointer to new table BL/BH = how many blocks/bytes per character CX/DX = number of characters/where to start in block AL = 1 BL = block to load the monochrome character set AL = 2 BL = block to load the double width character set AL = 3 BL = block to select related to attribute AL = 4 BL = block to load the 8x16 set (VGA) AL = 10h - 14h Same as above, but must be called after a set mode AL = 20h ES:BP pointer to a character table, using int 1Fh pointer BL = 0, DL = number of rows, 1=14rows, 2=25rows, 3=43rows CX = number of bytes per character in table AL = 22h use 8x14 character set, BL = rows AL = 23h use double width character set, BL = rows AL = 24h use 8x16 character set, BL = rows Get table pointers and other information AL = 30h ES:BP = returned pointer; CX = bytes/character; DL = rows BH = 0, get int 1Fh pointer, BH = 1, get int 43h pointer BH = 2, get 8x14 BH = 3, get double width BH = 4, get double width BH = 5, get mono 9x14 BH = 6, get 8x16 (VGA) BH = 7, get 9x16 (VGA) AH = 12h Miscellaneous functions, BL specifies sub-function number BL = 10h Get info, BH = 0, now color mode, 1 = now monochrome CH/CL = information bits/switches BL = 20h Set print screen to work with EGA/VGA Functions for VGA only (BL = 30-34h, AL returns 12h) BL = 30h Set number of scan lines, 0=200, 1=350, 2=400 BL = 31h AX = 0/1 Allow/Prevent palette load with new mode BL = 32h AL = 0/1 Video OFF/ON BL = 33h AL = 0/1 Grey scale summing OFF/ON BL = 34h AL = 0/1 Scale cursor size to font size OFF/ON BL = 35h Switch between adapter and motherboard video AL = 0, adapter OFF, ES:DX = save state area AL = 1, motherboard on AL = 2, active video off, ES:DX = save area AL = 3, inactive video on, ES:DX = save area BL = 36h, AL = 0/1 Screen OFF/ON AH = 13h Write Character string(cr,lf,bell and bs as operators) AL = 0/1 cursor NOT/IS moved, BL = attribute for all chars AL = 2/3 cursor NOT/IS moved, string has char/attr/char/attr BH = page number, 0 = 1st page, CX = number of characters DH/DL = row/column to start, ES:BP = pointer to string AH = 14h LCD Support AL = 0h ES:DI = pointer to font table to load BL/BH = which blocks/bytes per character CX/DX = number of characters/where to start in block AL = 1h BL = block number of ROM font to load AL = 2h BL = enable high intensity AH = 15h Return LCD information table Pointer in ES:DI, AX has screen mode AH = 16h GET/SET display type (VGA ONLY) AL = 0h Get display type BX = displays used, AL = 1Ah AL = 1h Set display type BX = displays to use, returns AL = 1Ah AH = 1Bh Get Video System Information (VGA ONLY) Call with BX = 0; ES:DI ptr to buffer area AH = 1Ch Video System Save & Restore Functions (VGA ONLY) AL = 0 Get buffer size AL = 1 Save system, buffer at ES:BX AL = 2 Restore system, buffer at ES:BX CX = 1 For hardware registers CX = 2 For software states CX = 4 For colors and DAC registers
0040:0000 Address of RS232 card COM1 0040:0002 Address of RS232 card COM2 0040:0004 Address of RS232 card COM3 0040:0006 Address of RS232 card COM4 0040:0008 Address of Printer port LPT1 0040:000A Address of Printer port LPT2 0040:000C Address of Printer port LPT3 0040:0010 Equipment bits: Bits 13,14,15 = number of printers 12 = game port attached 9,10,11 = number of rs232 cards 6,7 = number of disk drives 4,5 = Initial video mode (00=EGA, 01=CGA40,10=CGA80, 11=MONO) 3,2 = System RAM size 1 = Maths co-processor 0 = Boot from drives? 0040:0013 Main RAM Size 0040:0015 Channel IO size 0040:0017 Keyboard flag bits (byte) 7=ins, 6=caps, 5=num, 4=scrll, 3=ALT, 2=CTRL, 1=LSHFT, 0=RSHFT (toggle states) 0040:0018 Keyboard flag bits (byte) (depressed states) 0040:0019 Keyboard ALT-Numeric pad number buffer area 0040:001A Pointer to head of keyboard queue 0040:001C Pointer to tail of keyboard queue 0040:001E 15 key queue (head=tail, queue empty) 0040:003E Recalibrate floppy drive, 1=drive0, 2=drv1, 4=drv2, 8=drv3 0040:003F Disk motor on status, 1=drive0, 2=drv1, 4=drv2, 8=drv3 80h = disk write in progress 0040:0040 Disk motor timer 0=turn off motor 0040:0041 Disk controller return code 1=bad cmd, 2=no address mark, 3=cant write, 4=sector not found 8=DMA overrun,9=DMA over 64k 10h=CRC error,20h=controller fail, 40h=seek fail,80h=timeout 0040:0042 Disk status bytes (seven) 0040:0049 Current Video Mode (byte) 0040:004A Number of video columns 0040:004C Video buffer size in bytes 0040:004E Segment address of current video memory 0040:0050 Video cursor position page 0, bits8-15=row,bits0-7=column 0040:0052 Video cursor position page 1, bits8-15=row,bits0-7=column 0040:0054 Video cursor position page 2, bits8-15=row,bits0-7=column 0040:0056 Video cursor position page 3, bits8-15=row,bits0-7=column 0040:0058 Video cursor position page 4, bits8-15=row,bits0-7=column 0040:005A Video cursor position page 5, bits8-15=row,bits0-7=column 0040:005C Video cursor position page 6, bits8-15=row,bits0-7=column 0040:005E Video cursor position page 7, bits8-15=row,bits0-7=column 0040:0060 Cursor mode, bits 8-12=start line, 0-4=end line 0040:0062 Current video page number 0040:0063 Video controller base I/O port address 0040:0065 Hardware mode register bits 0040:0066 Color set in CGA mode 0040:0067 ROM initialisation pointer 0040:0069 ROM I/O segment address 0040:006B Unused interrupt occurrences 0040:006C Timer low count (every 55milliseconds) 0040:006E Timer high count 0040:0070 Timer rollover (byte) 0040:0071 Key-break, bit 7=1 if break key is pressed 0040:0072 Warm boot flag, set to 1234h for warm boot 0040:0074 Hard disk status byte 0040:0075 Number of hard disk drives 0040:0076 Head control byte for hard drives 0040:0077 Hard disk control port (byte) 0040:0078 - 7B Countdown timers for printer timeouts LPT1 - LPT4 0040:007C - 7F Countdown timers for RS232 timeouts, COM1 - COM4 0040:0080 Pointer to beginning of keyboard queue 0040:0082 Pointer to end of keyboard queue Advanced Video Data, EGA/VGA 0040:0084 Number of rows - 1 0040:0085 Number of pixels per character * 8 0040:0087 Display adapter options (bit3=0 if EGA card is active) 0040:0088 Switch settings from adapter card 0040:0089 - 8A Reserved 0040:008B Last data rate for diskette 0040:008C Hard disk status byte 0040:008D Hard disk error byte 0040:008E Set for hard disk interrupt flag 0040:008F Hard disk options byte, bit0=1 when using a single controller for both hard disk and floppy 0040:0090 Media state for drive 0 Bits 6,7=data transfer rate (00=500k,01=300k,10=250k) 5=two steps?(80tk as 40k) 4=media type 3=unused 2,1,0=media/drive state (000=360k in 360k drive) (001=360k in 1.2m drive) (010=1.2m in 1.2m drive) (011=360k in 360k drive) (100=360k in 1.2m drive) (101=1.2m in 1.2m drive) (111=undefined ) 0040:0091 Media state for drive 1 0040:0092 Start state for drive 0 0040:0093 Start state for drive 1 0040:0094 Track number for drive 0 0040:0095 Track number for drive 1 0040:0096 - 97 Advanced keyboard data 0040:0098 - A7 Real time clock and LAN data 0040:00A8 - FF Advanced Video data 0050:0000 Print screen status 00=ready,01=in progress,FFh=error
These modules read and write a dot to the video screen respectively. The programmer could retain the object code for these seperately, instead of placing them in a library, but the use of a library makes life simpler in that a library contains as many routines as you incorporate into it, thus simplifying the linking process. In other words, a library just groups object modules together under a common name.
The following programs describe the source code for the modules rdot() and wdot().
#include <dos.h>
union REGS regs;
wdot( int row, int column, unsigned int color )
{
regs.x.dx = row; regs.x.cx = column;
regs.h.al = color; regs.h.ah = 12;
int86( 0x10, ®s, ®s);
}
unsigned int rdot( int row, int column )
{
regs.x.dx = row; regs.x.cx = column;
regs.h.ah = 13; int86( 0x10, ®s, ®s);
return( regs.h.al );
}
The modules are compiled into object code. If we called the source
code VIDCALLS.C then the object code will be VIDCALLS.OBJ. To
create a library requires the use of the LIB.EXE program. This
is invoked from the command line by typing LIB
Enter in the name of the library you wish to create, in this case VIDCALLS. The program will check to see if it already exists, which it doesn't, so answer Y to the request to create it. The program requests that you now enter in the name of the object modules. The various operations to be performed are,
to add an object module +module_name to remove an object module -module_nameEnter in +vidcalls. It is not necessary to include the extension. The program then requests the list file generated by the compiler when the original source was compiled. If a list file was not generated, just press enter, otherwise specify the list file. That completes the generation of the VIDCALLS.LIB library.
The routines in VIDCALLS.LIB are incorporated into user programs as follows,
/* source code for program illustrating use of VIDCALLS.LIB */
#include <dos.h>
union REGS regs;
extern void wdot();
extern int rdot();
main()
{
int color;
regs.h.ah = 0; /* set up 320 x 200 color */
regs.h.al = 4;
int86( 0x10, ®s, ®s );
wdot( 20, 20, 2 );
color = rdot( 20, 20 );
printf("Color value of dot @20.20 is %d.\n", color);
}
The program is compiled then linked. When the linker requests
the library name, enter +VIDCALLS. This includes the rdot()
and wdot() modules at linking time. If this is not done, the linker
will come back with unresolved externals error on the rdot() and
wdot() functions.
; Microsoft C ; msc source; ; link source,,+vidcalls; ; ; TurboC ; tcc -c -ml -f- source.c ; tlink c0l source,source,source,cl vidcalls
#include <stdio.h> /* HACCESS1.C */
main()
{
char far *scrn = (char far *) 0xB8000000;
short int attribute = 164; /* Red on green blinking */
int full_screen = 80 * 25 * 2, loop = 0;
for( ; loop < full_screen; loop += 2 ) {
scrn[ loop ] = 'A';
scrn[ loop + 1 ] = attribute;
}
getchar();
}
The declaration of a far pointer specifies a 32 bit address. However
the IBM-PC uses a 20 bit address bus. This 20 bit physical address
is generated by adding the contents of a 16 bit offset to a SEGMENT
register.
The Segment register contains a 16 bit value which is left shifted four times, then the 16 bit offset is added to this generating the 20 bit physical address.
Segment register = 0xB000 = 0xB0000 Offset = 0x0010 = 0x 0010 Actual 20 bit address = 0xB0010When specifying the pointer using C, the full 32 bit combination of segment:offset is used, eg,
char far *memory_pointer = (char far *) 0xB0000000;Lets consider some practical applications of this now. Located at segment 0x40, offset 0x4A is the number of columns used on the current video screen. The following program shows how to access this,
main()
{
char far *video_parameters = (char far *) 0x00400000;
int columns, offset = 0x4A;
columns = video_parameters[offset];
printf("The current column setting is %d\n", columns);
}
A practical program illustrating this follows. It directly accesses
the video_parameter section of low memory using a far pointer,
then prints out the actual mode using a message.
main() /* HACCESS2.C */
{
static char *modes[] = { "40x25 BW", "40x25 CO", "80X25 BW",
"80X25 CO", "320X200 CO", 320X200 BW",
"640X200 BW", "80X25 Monitor" };
char far *video_parameters = (char far *) 0x00400000;
int crt_mode = 0x49, crt_columns = 0x4A;
printf("The current video mode is %d\n", modes[video_parameters[crt_mode]]);
printf("The current column width is %d\n", video_parameters[crt_columns]);
}
An adaptation of this technique is the following two functions.
Poke stores a byte value at the specified segment:offset,
whilst Peek returns the byte value at the specified segment:offset
pair.
void poke( unsigned int seg, unsigned int off, char value) {
char far *memptr;
memptr = (char far *) MK_FP( seg, off);
*memptr = value;
}
char peek( unsigned int seg, unsigned int off ) {
char far *memptr;
memptr = (char far *) MK_FP( seg, off);
return( *memptr );
}
The program COLOR illustrates the use of direct access
to update the foreground color of a CGA card in text mode. It
accepts the color on the command line, eg, COLOR RED will set
the foreground color to RED.
#include <string.h> /* Color.c */
#include <dos.h>
#include <stdio.h>
#include <conio.h>
#define size 80*25*2
struct table {
char *string;
int value;
} colors[] = { {"BLACK" , 0, "BLUE" , 1 },
{"GREEN" , 2, "CYAN" , 3 },
{"RED" , 4, "MAGENTA" , 5 },
{"BROWN" , 6, "LIGHT_GRAY" , 7 },
{"DARK_GRAY" , 8, "LIGHT_BLUE" , 9 },
{"LIGHT_GREEN" ,10, "LIGHT_CYAN" ,11 },
{"LIGHT_RED" ,12, "LIGHT_MAGENTA" ,13 },
{"YELLOW" ,14, "WHITE" ,15 } };
int getcolor( char *color) {
char *temp;
int loop, csize;
temp = color;
while( *temp )
*temp++ = toupper( *temp );
csize = sizeof(colors) / sizeof(colors[0]);
for( loop = 0; loop < csize; loop++ )
if( strcmp(color, colors[loop].string) == 0)
return(colors[loop].value;
return( 16 );
}
void setcolor( int attr ) {
int loop;
char far *scrn = (char far *) 0xb8000000;
for( loop = 1; loop < size; loop += 2 )
scrn[loop] = attr;
}
main( int argc, char *argv[] ) {
int ncolor;
if( argc == 2 ) {
ncolor = getcolor( argv[1] );
if( ncolor == 16 )
printf("The color %s is not available.\n", argv[1]);
else
setcolor( ncolor );
}
}
The following program illustrates the use of pointers to functions, in creating a simple shell program which can be used to specify the screen mode on a CGA system.
#include <stdio.h> /* Funcptr.c */
#include <dos.h>
#define dim(x) (sizeof(x) / sizeof(x[0]) )
#define GETMODE 15
#define SETMODE 0
#define VIDCALL 0X10
#define SCREEN40 1
#define SCREEN80 3
#define SCREEN320 4
#define SCREEN640 6
#define VID_BIOS_CALL(x) int86( VIDCALL, &x, &x )
int cls(), scr40(), scr80(), scr320(), scr640(), help(), shellquit();
union REGS regs;
struct command_table {
char *cmd_name;
int (*cmd_ptr) ();
}cmds[]={"40",scr40,"80",scr80,"320",scr320,"640",scr640,"HELP",help,"CLS",cls,"EXIT",shellquit};
cls() {
regs.h.ah = GETMODE; VID_BIOS_CALL( regs );
regs.h.ah = SETMODE; VID_BIOS_CALL( regs );
}
scr40() {
regs.h.ah = SETMODE;
regs.h.al = SCREEN40;
VID_BIOS_CALL( regs );
}
scr80() {
regs.h.ah = SETMODE;
regs.h.al = SCREEN80;
VID_BIOS_CALL( regs );
}
scr320() {
regs.h.ah = SETMODE;
regs.h.al = SCREEN320;
VID_BIOS_CALL( regs );
}
scr640() {
regs.h.ah = SETMODE;
regs.h.al = SCREEN640;
VID_BIOS_CALL( regs );
}
shellquit() {
exit( 0 );
}
help() {
cls();
printf("The available commands are; \n");
printf(" 40 Sets 40 column mode\n");
printf(" 80 Sets 80 column mode\n");
printf(" 320 Sets medium res graphics mode\n");
printf(" 640 Sets high res graphics mode\n");
printf(" CLS Clears the display screen\n");
printf(" HELP These messages\n");
printf(" EXIT Return to DOS\n");
}
get_command( char *buffer ) {
printf("\nShell: ");
gets( buffer );
strupr( buffer );
}
execute_command( char *cmd_string ) {
int i, j;
for( i = 0; i < dim( cmds); i++ ) {
j = strcmp( cmds[i].cmd_name, cmd_string );
if( j == 0 ) {
(*cmds[i].cmd_ptr) ();
return 1;
}
}
return 0;
}
main() {
char input_buffer[81];
while( 1 ) {
get_command( input_buffer );
if( execute_command( input_buffer ) == 0 )
help();
}
}