PS/2 Video Programming (MCGA)

The ins and outs of the PS/2 machines' video systems

Author: Richard Wilton
Source: Inside the IBM PCs - BYTE 1987 Extra Edition, page 67-72 (local copy)

Note: Pages 67-72 focus on the Multi-Color Graphics Array (MCGA). The VGA portion of the article begins on Page 72 and ends on Page 80. IMHO, there is more VGA programming stuff than you can shake a stick at, while MCGA doesn’t get much love. All typos are just that (I type out of self defense). -LFO

The IBM PS/2 series introduces two new video subsystems, the Multi-Color Graphics Array (MCGA) and Video Graphics Array (VGA). This article is an overview of the MCGA and the VGA from a programmer's point of view. If you are already familiar with older video adapters, such as the CGA, EGA, or Hercules cards, this article will point out the similarities and differences between the PS/2 video subsystems and previous IBM video adapters. If you are new to video hardware programming, you can use the examples in this article as a focus for further exploration of the PS/2 hardware.

Unlike the IBM PC, XT, and·AT, into which you must install a separate card that supports the necessary hardware to drive a video display, all the PS/2s are equipped with a built-in video subsystem on the motherboard. The Model 30 comes with the MCGA, while the Models 50, 60, and 80 use the VGA.

For compatibility (and, no doubt, in hopes of selling lots of hardware), IBM also offers a VGA adapter that implements the VGA subsystem on a card for the XT, AT, or PS/2 Model 30.


The MCGA and VGA differ from previous IBM video adapters in that both require that you use an analog monitor instead of a digital monitor. Adapters such as the CGA and the EGA use digital monitors in which the RGB color signals generated by the adapter are digital signals (on or off). This limits the number of different colors that the subsystem can display. For example, IBM's enhanced color display, which is driven by six RGB signals as generated by an EGA, can display a total of 64 (26) different colors. In contrast, the PS/2-compatible monitors use RGB signals with voltage levels that are continuously variable instead of simply on or off. Because the displayed brightness of a color corresponds to the voltage level of the color drive signals, an analog monitor can display a much larger variety of colors.

IBM offers one monochrome and two color monitors for use with the MCGA and the VGA. You can use a monochrome or color monitor with either video subsystem. You can also use EGA-compatible monitors with analog capability, such as the NEC MultiSync and Sony MultiScan monitors, with the MCGA and VGA.


Programs that run on the CGA can run unchanged on the MCGA, even if they bypass the video BIOS and program the hardware directly. Also, because the PS/2 Model 30 has a PC-compatible bus, a monochrome display adapter or Hercules adapter can coexist with the MCGA in the PS/2 Model 30.

The VGA is similar in its programming interface to the EGA. Its control ports and buffer addressing are EGA compatible, so programs that run on an EGA generally run on a VGA as well. The VGA is compatible enough with the EGA at the hardware level that the VGA can usually run ill behaved programs that access the EGA control registers directly.

From a programmer's perspective, there is not much resemblance between the MCGA and the VGA. The I/O port assignments in the MCGA and VGA differ significantly. So does the layout of video RAM in the two subsystems. A program that bypasses the video BIOS to control the hardware directly will probably not run on both the MCGA and the VGA unless it contains special code for programming each subsystem independently.


The programming interface to PS/2 video hardware is documented in the IBM technical reference manuals for the Models 30, 50, and 60. The video BIOS is covered by a separate set of IBM reference manuals, the Personal System/2 and Personal Computer BIOS Interface Technical Reference. Obviously, this article does not cover all the details of the video hardware implementation. If you need to understand the hardware or firmware in detail, you should obtain the appropriate IBM technical manuals.


The heart of the MCGA circuitry lies in two proprietary gate arrays: the memory-controller gate array, which incorporates the functions of a CRT controller, and the video-formatter gate array, which controls video mode selection and color-attribute decoding.

Reg. Function
0 Horizontal total
1 Horizontal displayed
2 Start horizontal sync
3 Sync pulse width
4 Vertical total
5 Vertical total adjust
6 Vertical displayed
7 Start vertical sync
8 (Reserved)
9 Scan lines per character
0A Cursor start
0B Cursor end
0C Start address high
0D Start address low
0E Cursor location high
0F Cursor location low
10 Mode control
11 Interrupt control
12 Character generator, sync polarity
13 Character-generator pointer
14 Character-generator count
Table 1: MCGA memory-controller registers.
Registers 0 through 0F hexadecimal are comparable to those in the CGA 's CRT controller

You can program the memory controller through a set of 8-bit registers (see Table 1) mapped to I/O ports 3D4 and 3D5 hexadecimal. (Ed. For the remainder of this article, addresses will be in hexadecimal.) As on the CGA, you access the registers by first writing the register number to the port at 3D4, and then writing or reading the specified register at 3D5. Unlike the CGA, however, you can read and write all the memory controller registers. This is a handy feature if you are debugging programs, although it's not a good idea to rely on it if you are concerned about maintaining CGA compatibility.

The first 16 memory-controller registers are analogs of the registers on the Motorola 6845, the CRT controller chip used in the CGA. This means that CGA compatible programs that access these registers directly can also run on the MCGA. Because the default horizontal and vertical CRT timing parameters used on the MCGA differ from those used on the CGA, you might want to write-protect the first seven registers so that CGA compatible programs that attempt to update these registers do not inadvertently disrupt crucial CRT timing signals. Bit 7 of the mode-control register (register 10h) is the write-protect bit for the timing registers.

The remaining memory-controller registers control video mode selection and the alphanumeric character generator. These registers do not exist on the CGA. They support functions that are similar to what is available on the VGA: additional graphics modes and RAM-loadable alphanumeric character sets.

The video formatter supports three CGA-compatible control registers. The mode-control register (I/O port 3D8) controls video mode selection. The color-control register (port 3D9) controls palette and graphics mode background-color selection. The status register (port 3DA) is a read-only register whose contents indicate the status of the CRT's horizontal and vertical timing signals. All three of these registers are compatible with the analogous registers on the CGA.

In addition to the six video modes supported by the CGA, the MCGA offers a 640 by 480 two-color graphics mode (video BIOS mode llH) and a 320 by 200 256-color graphics mode (BIOS mode 13H). You can set up both new modes, as well as all the CGA-compatible modes using INT l0h function 0 (see Listing 1).

mov   ah,0                ; AH = INT 10h function #
mov   al, VideoModeNumber ; AL = 11h (640x480 two-color)
                          ; 12h (640x480 16-color)
                          ; 13h (320x200 256-color)
int   10h                 ; Call video BIOS
Listing 1: Video mode selection using the video BIOS.

New Features

Two features of the MCGA are of special interest to programmers. One is that the vertical resolution of both alphanumeric and graphics modes is greater than on previous IBM video adapters. The other is that the MCGA can display up to 256 different colors at one time out of a possible 262,144 (256K) colors.

Mode # Description MCGA VGA
0 40 by 25 16-color alphanumeric
(320 by 400 resolution)
0 40 by 25 16-color alphanumeric
(360 by 400 resolution)
2 80 by 25 16-color alphanumeric
(640 by 400 resolution)
2 80 by 25 16-color alphanumeric
(720 by 400 resolution)
11H 640 by 480 two-color graphics X X
12H 640 by 480 16-color graphics X
13H 320 by 200 256-color graphics X X
Table 2: New BIOS video modes on the MCGA and VGA

The vertical resolutions of the default BIOS video modes are listed in Table 2. In alphanumeric modes, the vertical resolution is 400 scan lines-twice that of the CGA and better than the EGA's 350-line "enhanced" modes. Since the BIOS still displays 25 rows of characters in alphanumeric modes, the vertical size of each displayed character is 16 scan lines. These higher-resolution characters are sharp and easy to read.

When the MCGA emulates the CGA graphics modes (640 by 200 two-color and 320 by 200 four-color), it doubles the vertical size of pixels so that each is two scan lines high. Thus, although the CGA compatible graphics modes use the same resolution in terms of pixels, the displayed resolution is still 400 lines, so these modes have a sharper appearance on the MCGA than they do on a CGA.

Another feature of both the MCGA and the VGA is expanded color display capability. This is provided by a digital-to-analog converter (DAC) that generates the analog RGB signals used to drive the PS/2 monochrome and color monitors. (The monochrome monitor responds only to the green color signal; the color monitors recognize all three.)

The video DAC uses a set of 256 eighteen-bit internal registers, each of which specifies an RGB combination. Each of the three primary colors is allotted 6 bits of each color register; the DAC converts each 6-bit value to a corresponding analog voltage level in the signals it outputs to the monitor. Thus, the video DAC can produce any of 64, color intensities for each of the three primary colors in a color register, thereby generating 256K (643) color combinations. Since there are 256 video DAC color registers, the video subsystem can display any 256 of the 256K color possibilities at one time.

Video BIOS

The video BIOS on the Model 30 provides the same set of functions as the motherboard ROM BIOS on the PC. The programming interface is the same: You access all video BIOS functions through interrupt l0h and pass parameters to the BIOS routines in the CPU's registers.

Function 10h: Color-palette interface
AL=3 Toggle alphanumeric intensity/blink state
AL=7 Read individual palette register (VGA only)
AL=8 Read overscan (border color) register (VGA only)
AL=9 Read all palette registers and overscan register (VGA only)
AL=10h Set individual video DAC color register
AL=12h Set block of video DAC color registers -
AL=13h Select video DAC color page (VGA only)
AL=15h Read individual video DAC color register
AL=17h Read block of video DAC color registers
AL=1Ah Read video DAC color-page state (VGA only)
AL=1Bh Perform gray-scale summing
Function 11h: Character-generator interface
AL=4 Load 8 by 16 alphanumeric characters
AL=14h Set alphanumeric mode using 8 by 16 characters
AL=24h Load 8 by 16 graphics characters
Function 12h: Alternate select
BL=30h Select vertical resolution for alphanumeric modes (VGA only)
BL=31h Enable/disable default palette loading
BL=32h Enable/disable video addressing
BL=33h Enable/disable default gray-scale summing
BL=34h Enable/disable alphanumeric cursor emulation (VGA only)
BL=35h Display-switch interface
Function 1Ah: Display combination code
AL=0 Read display combination code
AL=1 Write display combination code
Function 1Bh: Functionality/state information
Function 1Ch: Save/restore video state (VGA only)
AL=0 Return state buffer size
AL=1 Save video state
AL=2 Restore video state
Table 3: New INT 10h functions on the MCGA and VGA.

Also, several new INT 10h functions are available in the Model 30, as well as the other models in the PS/2 series (see Table 3). IBM has expanded the INT 10h function l0h to provide access to the video DAC color registers. Function 12h has several new subfunctions that let you vary the default actions of other BIOS routines. For example, you can call INT 10h function 12h with the BL register set to 31h to enable or disable default palette loading when the video mode is changed.

INT 10h functions lAh and lBh are new to the PS/2 series. Your programs can call these INT 10h functions to determine the state of the video subsystem. A call to function 1Ah returns the video subsystem's display combination code, which indicates what type of monitor is in use. Function lBh returns a table whose contents describe the current state of the video BIOS: the current video mode, active video page, amount of video RAM available, and so on. These functions are useful in programs designed to run in more than one video mode, as well as in pop-up RAM-resident programs that must determine the current video state to produce appropriate video output.

Alphanumeric-Mode Programming

Despite the MCGA's improved resolution, programming in alphanumeric modes is virtually the same as on the CGA. The important difference is that the alphanumeric character generator on the MCGA can display user-defined characters. (The EGA, VGA, Hercules Graphics Card Plus, and Hercules InColor Card also have this capability.) The MCGA's alphanumeric character generator can display characters from any of four different 256-character tables defined in video RAM.

To make the MCGA's character generator display one of these character sets, you first load the bit patterns that define the characters into video RAM. Then you program the character generator to copy the bit patterns from video RAM into one of its two internal character-definition tables. (These character-definition tables are called font pages in the IBM technical literature.)

; load video BIOS 8x8 characters into alphanumeric character generator
mov   ax, 1102h                  ; AH = INT 10h function number
                                 ; AL = 8x8 character-set load
mov   bx, 0                      ; BX = block to load
int   10h                        ; load 8x8 characters into RAM
mov   ax, 1103h                  ; AH = INT 10h function number
                                 ; AL = character-generator load
mov   bx, 0                      ; BX = blocks to load
int   10h                        ; load 8x8 characters into
                                 ; character generator
; program CRT controller to display 8x8 characters
mov   dx, 3D4h                   ; DX = MCGA I/O port address
mov   ax, 309h                   ; AL = 9 (register number)
                                 ; AH = 3 (value for register)
out   dx, ax                     ; update scan-lines register
mov   al, 0Ah                    ; AL = 0Ah (register number)
out   dx, ax                     ; update cursor-start register
mov   al, 0Bh                    ; AL = 0Bh (register number)
out   dx, ax                     ; update cursor-end register
; update status variables in video BIOS data segment
mov   ax, 40h
mov   ds, ax                     ; DS -> video BIOS data segment
mov   word ptr ds:[4Ch], 80*50*2 ; update CRT_LEN
mov   byte ptr ds:[84h], 49      ; update ROWS
mov   word ptr ds:[85h], 8       ; update POINTS
Listing 2: Establishing an 80 by 50 alphanumeric mode on the MCGA

It is easy to use the MCGA's RAM-loadable character sets on the MCGA to display characters that are smaller than the default 16-scan-line characters. In Listing 2, the program calls the video BIOS to load the default graphics-mode character definitions-in which characters are only eight lines high-for use by the alphanumeric character generator. The code then reprograms the MCGA's memory controller to display only eight scan lines in each row of characters; the result is 50 rows of 80 characters each.

Apart from supporting RAM-loadable character sets, the MCGA replicates almost all the CGA's capabilities. However, the MCGA is not troubled by problems with display interference in alphanumeric modes. On the CGA, you must carefully synchronize CPU accesses to the video RAM with horizontal and vertical retrace intervals in the display refresh cycle. If you don't, you might see random patterns of interference or snow on the screen each time a program accesses the video buffer. The hardware design of the MCGA is such that this sort of display interference does not occur.

Surprisingly, the MCGA cannot generate a colored border. In alphanumeric modes on the CGA, you can display a border in any of 16 colors selected by programming the color-select register (I/O port 3D9). On the MCGA, you can still program the color-select register, but the MCGA does not display a border, regardless of the value you store in the register.

Graphics-Mode Programming

In CGA-compatible 640 by 200 two color and 320 by 200 four-color modes, the MCGA emulates the CGA. The system maps pixels with a two-way interleave in the video buffer at B800:0000, just as they are mapped on the CGA. Video-buffer addressing is different, however, in 640 by 480 two-color and 320 by 200 256-color modes.

Figure 1: A video-buffer map in 640 by 480 two-color graphics mode

The video-buffer map in both 640 by 480 two-color and 320 by 200 256-color modes starts at A000:0000. Both modes map pixels linearly in the buffer from left to right and from top to bottom on the screen. In 640 by 480 two-color mode, a bit represents one pixel, so there are eight pixels to each byte in the video buffer and 80 bytes in the buffer per row of pixels on the screen (see Figure 1).

Figure 2: A video-buffer map in 320 by 200 256-color graphics mode.

In 320 by 200 256-color mode, each pixel value comprises 8 bits, so the video buffer is mapped as 200 320-byte rows (see Figure 2).

sp13 PROC near
; call with: AX = y coordinate
;            BX = x coordinate
;            CL = pixel value
; compute address of pixel in video buffer
mov   dx,320
mul   dx
add   bx, ax      ; BX = X + 320*Y
mov   ax, 0A000h
mov   ds, ax      ; DS : BX -> pixel in video buffer
; update the pixel value
mov   [bx], cl
sp13 ENDP
Listing 3: Setting the value of a pixel in 320 by 200 256-color mode.

You can write routines that manipulate pixels in these MCGA graphics modes by modifying code that runs in CGA-compatible graphics modes. The routine in Listing 3 is an example of code that up-dates a single pixel in 320 by 200 256-color mode. The program computes the video-buffer address by multiplying the number of pixels in each row by the pixel y coordinate and then adding the pixel x coordinate. Since each byte in the buffer represents one pixel, updating a pixel consists of a single machine instruction.

The MCGA's 640 by 480 two-color graphics mode deserves attention because its horizontal resolution and vertical resolution are the same in terms of the number of pixels displayed per inch. (Programmers sometimes describe this circumstance by saying that the pixels are "square") This means that when you draw a figure in 640 by 480 graphics mode, you do not need to scale the figure to accommodate different horizontal and vertical resolutions.

Video DAC Programming

Programming the MCGA's video DAC is straightforward when you use the video BIOS. In all modes except the 320 by 200 256-color graphics mode, you can use only the first 16 video DAC registers. The video BIOS loads these registers by default with a set of 16 CGA-compatible color values. You can, however, update any of these color registers using any of the 256K color combinations available.

mov   ah, 10h        ; AH = INT 10h function #
mov   bx, 7          ; BX = 7 (register #)
mov   dh, RedValue   ; DH, CH, CL = 6-bit RGB values
mov   ch, GreenValue
mov   cl, BlueValue
int   10h            ; Call video BIOS
Listing 4: Updating a video DAC color register

For example, Listing 4 shows how you could change the color value in video DAC register 7. The color-register value is actually 18 bits in size-red, green, and blue components are each 6 bits. The higher the value you specify for each component, the higher the displayed intensity of that color.

If a monochrome display is attached to the MCGA, the video BIOS performs a gray-scaling computation before it loads a color value into the specified video DAC color register. The video BIOS performs gray-scaling by taking a weighted average of the red, green, and blue values you specify. (The formula used is 30 percent red + 59 percent green + 11 percent blue.) The result is a gray-scale value that corresponds to the overall intensity of the specified color combination.

Richard Wilton (6236 West Sixth St., Los Angeles, CA 90048) is author of The Programmer's Guide to PC and PS/2 Video Systems; which is due out in November

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