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- The program INT 70h is a very simple driver for the
- hercules graphics card (aka the HGC). It is intended to ease
- the use of the graphics while still allowing full use of the
- card at machine language speed. It even has its own language
- for accessing the different functions.
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- There are only 8 commands in the INT 70h language:
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- 0 Stop the interupt program
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- 1 Turn graphics on
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- 2 Turn text on
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- 3 Draw a point
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- 4 Draw a line
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- 5 Set the active page
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- 6 Clear the given page
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- 7 Fill the given page
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- Since there are only 7 instructions, I use the four most
- significant bits in a byte to specify a function in a command
- byte. The four least significant bits specify a page of
- memory in some functions (the hercules graphics card has two
- graphics/text pages).
-
- The language is a very simple one. You set a byte in
- memory to a value as specified above. If it needs parameters
- (i.e. screen coordinates for a point), you use the next few
- WORDS of memory to specify them. You use memory words (2
- bytes) because a screen coordinate may be more than 255 which
- is as large a value as a byte can handle. One last important
- note:
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- Always follow your program code by a
- byte value of 0 in the command byte
- position. This tells the interrupt when
- to stop executing your code.
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- This is an example of what your code should look like:
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- ┌─────────────────────────────────────────────────────┐
- (1) │ DB 11h ; turn on the graphics mode │
- │ ; (page 1) │
- (2) │ DB 51h ; set active page │
- (3) │ DB 42h ; draw a line using XOR │
- (4) │ DW 10, 20, 40, 80 ; from (10,20) to (40,80) │
- └─────────────────────────────────────────────────────┘
-
- line (1): 10h ---> turn on graphics
- + 1h ---> page 1 (you can use page 0 or 1)
- ----
- 11h
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- line (2): 50h ---> set active page (for plotting points
- and lines)
- + 1h ---> page 1 is active
- ----
- 51h
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- line (3): 40h ---> draw a line
- + 2h ---> use the XOR method
- ----
- 42h
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- line (4): 10, 20, 40, 80 ---> words (specified w/ DW
- instead of DB)
-
- So, this program would turn on the graphics page, without
- clearing it and plot a line using an exclusive or method.
-
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- To call this interrupt, you must do two things:
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- 1) put the address in the AX & BX registers
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- 2) use the instruction INT 70h
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- The address in AX & BX is the segment where the code is being
- kept is put into AX and the offset within that segment is put
- into BX. This can be accomplished in one of two ways:
-
-
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- MOV AX, SEG <codename>
- MOV BX, OFFSET <codename>
- INT 70h
-
- or
-
- MOV AX, DS
- LEA BX, <codename>
- INT 70h
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- Functions:
- number of purpose of
- number name parameters parameters
- ────── ──────────────── ────────── ─────────────
- 10h Turn on graphics 0 *
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- 20h Turn on text 0 *
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- 30h Draw a point 2 ^ X & Y coordinates
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- 40h Draw a line 4 ^ X&Y coordinates
- of both end point
- 50h Set the active pg 0 *
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- 60h Clear page 0 *
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- 70h Fill page 1 * value to fill with
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- ^ -- this means that the type of plotting used is in the four
- least significant bits:
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- 1: turn the points chosen on
-
- 2: use exclusive or to plot the points
- (i.e. if the point is on, turn it
- off. if it is off, turn it on.)
-
- 3: turn the points chosen off
-
-
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- * -- this means that the page number for the function should
- be in the four least significant bits
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- 0: page 0 (the page it usually is on when you
- start)
- 1: page 1 (used by the CGA card)
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- This is an example of a full MASM program using INT 70h:
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- CSEG SEGMENT
- ORG 100h
- ASSUME CS:CSEG, DS:CSEG, SS:CSEG
-
- START: MOV AX, CS ; this gets the segment value
- LEA BX, Gr_code ; this gets the offset
- INT 70h ; call the graphics interupt
-
- MOV AH, 7 ; DOS fn to get a key
- INT 20h ; call the DOS interupt
-
- MOV AX, CS ; get the segment value
- LEA BX, Reset ; get the offset
- INT 70h ; call the graphics interupt
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- INT 20h ; end the program
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- Gr_code DB 11h ; turn on graphics (page 1)
- DB 61h ; clear page 1
- DB 51h ; make page 1 the active page
-
- DB 31h ; put a point (using OR)
- DW 5, 5 ; at (5, 5) on the screen
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- DB 42h ; draw a line (using XOR)
- DW 10, 20, 40, 80 ; from (10, 20) to (40, 80)
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- DB 32h ; switch a point (using XOR)
- DW 6, 5 ; at (6, 5)
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- DB 33h ; turn off a point (mask using
- AND)
- DW 5, 5 ; at (5, 5)
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- DB 0 ; signify end of code
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- Reset DB 20h ; reset to text mode
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- DB 70h ; fill page 0 with . . .
- DB 32, 7 ; spaces with normal attributes
- DB 0 ; signify end of code
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- CSEG ENDS
- END START
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