Early microcomputers included varying degrees of graphics presentation, mostly of fairly low resolution. Some were programmed to drive CalComp plotters, just as had been done with earlier minicomputers, such as the PDP-8. With the introduction of the IBM/PC, a standard for graphics presentation was established which was widely disseminated, beginning with the CGA video board.
At first, one of the interrupts of the INTEL 8088, interrupt 10, was dedicated to communication with the video monitor, which was arranged for two modes of operation, graphics and text. The reason for the latter was a matter of bandwidth; the use of a character generator driven by ASCII characters was much better than any other system for placing dots in the graphics bitmap. As a consequence, the ROM BIOS underlying the MS/DOS operating system contained a provision for a mixture of approximately sixteen text and graphics primitives.
Early C compilers offered as close an approximation to these sixteen primitives as was possible within the syntax of the language. Later on, more elaborate graphic functions were included within the runtime libraries, corresponding to a certain perception of users requirements, or simply of their imagined desires.
From the point of view of developing an integrated graphics environment, it turns out that the first offerings were the best; fortunately they already contained direct access to individual pixels, which was an adequate foundation upon which to build a graphics package.
The entire set of functions consisted of videobackground - set the color of the frame
Only the items shown in boldface were ever used to write programs such as GEOM, and of those, videodot was the essential function for creating computer graphics. The principal problem in changing over to Objective C lies in substituting printf, which is not a video function at all, but nevertheless uses one of the BIOS interrupts to put text on the screen, irrespective of whether it is in text or graphics mode. It is this detail especially, which complicates any simple substitution of video functions when changing from one operating system to another.
The following comprises the header file which has always been incorporated in TURBO C programs to make them compatible with WIZARD C programs, making the use of any TURBO C functions unnecessary. Such a substitution is feasible because both C compilers still create code for MS/DOS, in which the BIOS and screen interrupts are the same.
As for the code, one places the whole interrupt-10 package in its own .obj file. The file is relatively small, inasmuch as the functions merely transfer arguments between the stack and the registers of the CPU, conforming to differences between assembly language and C's stack protocol.
/* */
/* VIDEOH.H: Function prototypes */
/* */
/* By : Saturnino Julio De La Trinidad H. */
/* */
/* Departamento de Aplicacion de Microcomputadoras */
/* Instituto de Ciencias de la Universidad Autonoma de Puebla */
/* */
#if __STDC__
#define _Cdecl
#else
#define _Cdecl cdecl
#endif
#if !defined(__WIZARD_DEF_)
#define __WIZARD_DEF_
void far _Cdecl videobackground(int color);
void far _Cdecl videocattr(int page, char byte, char attr, int count);
void far _Cdecl videochar(int page, char byte, int count);
void far _Cdecl videocmode(int startrow, int endrow);
void far _Cdecl videocursor(int page, int row, int column);
void far _Cdecl videodot(int pixelrow, int pixelcolumn, int color);
void far _Cdecl videomode(int mode);
void far _Cdecl videopage(int page);
void far _Cdecl videopalette(int colorset);
void far _Cdecl videoputc(char byte, int color);
void far _Cdecl videoscroll(int ulrow, int ulcol, int lrrow, int lrcol,
int lines, int attr);
int far _Cdecl videogchar(int page);
int far _Cdecl videogcmode(void);
int far _Cdecl videogcols(void);
int far _Cdecl videogcursor(int page);
int far _Cdecl videogdot(int pixelrow, int pixelcolumn);
int far _Cdecl videogmode(void);
int far _Cdecl videogpage(void);
struct pen {
char charrow;
char charcolumn;
unsigned char pixelrow;
unsigned char pixelcolumn;
};
int far _Cdecl videogpen(struct pen *penv);
#endif