Q12010 - Directly Accessing Video Memory from a C Program

Directly Accessing Video Memory from a C Program

Q12010

6.00 6.00a 6.00ax 7.00 | 1.00 1.50 MS-DOS | WINDOWS kbprg ---------------------------------------------------------------------- The information in this article applies to: - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0 and 1.5 ---------------------------------------------------------------------- SUMMARY ======= The text below presents a sample C function that demonstrates directly accessing video memory addressed starting at segment B800 in the MS-DOS environment. MORE INFORMATION ================ When an application directly accesses a particular memory location, it becomes less portable, and may not run as expected on another machine or on a future machine. Given this warning, the code example below directly accesses video memory. Sample Code ----------- /* * Compile options needed: None */ // Video.c -- Function to place a character and its standard // attribute into the desired video memory page. // // Note: This function assumes that the display is set to // color or monochrome, 80-column, text mode. #include <dos.h> #define MAKELONG(a, b) ((long)(((unsigned)a) \ | ((unsigned long)((unsigned)b)) << 16)) #define COLORTEXT_BUFFER 0XB800 void video(int pageno, int row, int col, char *ch, char attrib) // pageno : page number to load character into (0 to 3) // row : row of location 0 to 24 // col : column of location 0 to 79 // ch : character to be placed there // attrib : standard character attribute { unsigned int offset; // Offset from the segment address of // the desired video page char far *y; // Long pointer to the position in memory // where we will put the character and // its attribute (next byte) // Calculate the in-page offset with // page number offset and segment address offset = (unsigned int) ((row * 160 ) + (col * 2) + (pageno * 4096)); // Set the character. y = (char far *)MAKELONG(offset, COLORTEXT_BUFFER); *y = *ch; // Set the attribute byte. See an MS-DOS Programmer's // Reference manual for more information on video attributes. offset++; y = (char far *)MAKELONG(offset, COLORTEXT_BUFFER); *y = attrib; } Additional reference words: kbinf 1.00 1.50 6.00 6.00a 6.00ax 7.00 KBCategory: kbprg KBSubcategory: CLngIss

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