- CHAPTER 8 -

LINE-A

Overview

The Line-A portion of the operating system is so named because it uses a special exception vector of 680x0 processors triggered when the first nibble of the a command word is $A. On Atari systems this vector is routed to the operating system ROMs and provides a low-level yet high-speed graphics interface.

The Line-A system is included in this document for completeness only. It is recommended that its use be avoided and that the counterpart VDI calls be used instead. Atari has not guaranteed that it will maintain Line-A compatibility in future systems. Its functionality has already been limited as video capabilities have advanced beyond its design.

The Line-A Variable Table

The Line-A opcode $A000 will return a pointer to an internal variable table in D0 and A0. This table is used by the Line-A functions as a parameter passing mechanism as opposed to using the stack or internal registers.

Members of the Line-A variable table are accessed via offsets from the base address. The function, location, and size of documented variables are as follows:

Name
Offset
Size
Contents
RESERVED
-910
LONG
Reserved for future use.
CUR_FONT
-906
LONG
Pointer to the current font header.
RESERVED
-902
92 BYTEs
Reserved for future use.
M_POS_HX
-856
WORD
X Offset into the mouse form of the 'hot spot'.
M_POS_HY
-854
WORD
Y Offset into the mouse form of the 'hot spot'.
M_PLANES
-852
WORD
Writing mode for the mouse pointer (1 = VDI Mode, -1 = XOR Mode). Defaults to VDI mode.
M_CDB_BG
-850
WORD
Mouse pointer background color.
M_CDB_FG
-848
WORD
Mouse pointer foreground color.
MASK_FORM
-846
32 WORDs
Image and Mask for the mouse pointer. Data is stored in the following format: Line 0 Mask

Line 0 Image

Line 1 Mask

Line 1 Image

etc.
INQ_TAB
-782
46 WORDs
This area contains 45 WORDs of information returned from a vq_extnd() of the physical screen workstation plus one extra reserved WORD.
DEV_TAB
-692
46 WORDs
This area contains the first 45 WORDs of information returned from a v_opnwk() of the physical screen workstation plus one extra reserved WORD.
GCURX
-602
WORD
Current mouse pointer X position.
GCURY
-600
WORD
Current mouse pointer Y position.
M_HID_CT
-598
WORD
Current mouse 'hide' count (number of times mouse has been hidden, 0 = visible).
MOUSE_BT
-596
WORD
Bitmap of the current mouse button status.
REQ_COL
-594
48 WORDs
Contains 48 WORDs of RGB data for the first 16 VDI color registers as would be returned by vq_color().
SIZ_TAB
-498
15 WORDs
This table contains the final 12 WORDs of information returned from a v_opnwk() of the physical screen workstation plus 3 reserved WORDs.
RESERVED
-468
WORD
Reserved for future use.
RESERVED
-466
WORD
Reserved for future use.
CUR_WORK
-464
LONG
Pointer to the current VDI workstation attribute table.
DEF_FONT
-460
LONG
Pointer to the default font header.
FONT_RING
-456
4 LONGs
This area contains three pointers and a NULL. The first two pointers point to linked lists of system font headers. The third pointer points to the linked list of GDOS based fonts.
FONT_COUNT
-440
WORD
Total number of fonts pointed to by the FONT_RING pointers.
RESERVED
-438
90 BYTEs
Reserved for future use.
CUR_MS_STAT
-348
BYTE
Bitmap of mouse status since the last interrupt as follows: Bit Meaning

0 Left mouse status (0=up)

1 Right mouse status (0=up)

2 Reserved

3 Reserved

4 Reserved

5 Mouse move flag (1=moved)

6 Right mouse status flag

(0=hasn't changed)

7 Left mouse status flag

(0=hasn't changed)
RESERVED
-347
BYTE
Reserved for future use.
V_HID_CNT
-346
WORD
Number of times the text cursor has been hidden (0 = visible).
CUR_X
-344
WORD
X position where mouse pointer will be drawn.
CUR_Y
-342
WORD
Y position where mouse pointer will be drawn.
CUR_FLAG
-340
BYTE
Mouse redraw flag (if non-zero, mouse pointer will be redrawn at the next vertical blank interrupt).
MOUSE_FLAG
-339
BYTE
Mouse interrupt flag (0=disable interrupts)
RESERVED
-338
LONG
Reserved for future use.
V_SAV_XY
-334
2 WORDs
X and Y position of the text cursor as saved by the VT-52 emulator.
SAVE_LEN
-330
WORD
Height of the form saved in SAVE_AREA in pixels.
SAVE_ADDR
-328
LONG
Address of the first WORD of screen data contained in SAVE_AREA.
SAVE_STAT
-324
LONG
Save status flag as follows: Bit Meaning

0 Save buffer valid? (0=no)

1 Width of save

(0=16 bits, 1=32 bits)
SAVE_AREA
-322
256 BYTEs
Save buffer for the mouse pointer,
USER_TIM
-66
LONG
Pointer to a routine which occurs at each timer tick. (use vex_timv() instead). Routine ends by jumping to function pointed to by NEXT_TIM.
NEXT_TIM
-62
LONG
See above.
USER_BUT
-58
LONG
Pointer to a routine called each time a mouse button is pressed (use vex_butv() instead).
USER_CUR
-54
LONG
Pointer to a routine called each time the mouse needs to be rendered (use vex_curv() instead).
USER_MOT
-50
LONG
Pointer to routine called each time the mouse is moved (use vex_motv() instead).
V_CEL_HT
-46
WORD
Current text cell height.
V_CEL_MX
-44
WORD
Number of text columns - 1.
V_CEL_MY
-42
WORD
Number of text rows - 1.
V_CEL_WR
-40
WORD
Number of bytes between character cells.
V_COL_BG
-38
WORD
Text background color.
V_COL_FG
-36
WORD
Text foreground color.
V_CUR_AD
-34
LONG
Text cursor physical address.
V_CUR_OF
-30
WORD
Offset (in bytes) from physical screen address to the top of the first text character.
V_CUR_XY
-28
2 WORDs
X and Y character position of the text cursor.
V_PERIOD
-24
BYTE
Current cursor blink rate.
V_CUR_CT
-23
BYTE
Countdown timer to next blink.
V_FNT_AD
-22
LONG
Pointer to system font data (monospaced).
V_FNT_ND
-18
WORD
Last ASCII character in font.
V_FNT_ST
-16
WORD
First ASCII character in font.
V_FNT_WD
-14
WORD
Width of the system font form in bytes.
V_REZ_HZ
-12
WORD
Horizontal pixel resolution.
V_OFF_AD
-10
LONG
Pointer to font offset table.
RESERVED
-6
WORD
Reserved for future use.
V_REZ_VT
-4
WORD
Vertical pixel resolution.
BYTES_LIN
-2
WORD
Bytes per screen line.
PLANES
0
WORD
Number of planes in the current resolution.
WIDTH
2
WORD
Width of the destination form in bytes.
CONTRL
4
LONG
Pointer to the CONTRL array.
INTIN
8
LONG
Pointer to the INTIN array.
PTSIN
12
LONG
Pointer to the PTSIN array.
INTOUT
16
LONG
Pointer to the INTOUT array.
PTSOUT
20
LONG
Pointer to the PTSOUT array.
COLBIT0
24
WORD
Color bit value used for plane 0.
COLBIT1
26
WORD
Color bit value used for plane 1.
COLBIT2
28
WORD
Color bit value used for plane 2.
COLBIT3
30
WORD
Color bit value used for plane 3.
LSTLIN
32
WORD
Last pixel draw flag (0=draw, 1=don't draw). Used to prevent the last pixel in a polyline segment drawn in XOR mode from overwriting the first pixel in the next line.
LNMASK
34
WORD
Line draw pattern mask.
WMODE
36
WORD
VDI writing mode.
X1
38
WORD
X coordinate for point 1.
Y1
40
WORD
Y coordinate for point 1.
X2
42
WORD
X coordinate for point 2.
Y2
44
WORD
Y coordinate for point 2.
PATPTR
46
LONG
Fill-pattern pointer.
PATMSK
50
WORD
This value is AND'ed with the value in Y1 to give an index into the current fill pattern for the current line.
MFILL
52
WORD
Multiplane fill pattern flag (0=Mono).
CLIP
54
WORD
Clipping flag (0=disabled).
XINCL
56
WORD
Left edge of clipping rectangle.
XMAXCL
58
WORD
Right edge of clipping rectangle.
YMINCL
60
WORD
Top edge of clipping rectangle.
YMAXCL
62
WORD
Bottom edge of clipping rectangle.
XDDA
64
WORD
Text scaling accumulator (set to $8000 prior to blitting text).
DDAINC
66
WORD
Scaling increment. If SIZE1 is the actual point size and SIZE2 is the desired point size then to scale up use:To scale down use:
SCALDIR
68
WORD
Text scaling direction (0=down, 1=up).
MONO
70
WORD
Monospaced font flag.
SOURCEX
72
WORD
X coordinate of character in font form.
SOURCEY
74
WORD
Y coordinate of character in font form.
DESTX
76
WORD
X position on screen to output character at.
DESTY
78
WORD
Y position on screen to output character at.
DELX
80
WORD
Width of the character to output.
DELY
82
WORD
Height of the character to output.
FBASE
84
LONG
Pointer to the font character image block.
FWIDTH
88
WORD
Width of the font form in bytes.
STYLE
90
WORD
Special effects flag bitmap as follows: Bit Meaning

0 Thickening

1 Lightening

2 Skewing

3 Underlining

(not supported by Line-A)

4 Outlining
LITEMASK
92
WORD
Mask to lighten text (usually $5555).
SKEWMASK
94
WORD
Mask to skew text (usually $5555).
WEIGHT
96
WORD
Width to thicken characters by.
ROFF
98
WORD
Offset above baseline used for italicizing.
LOFF
100
WORD
Offset below baseline used for italicizing.
SCALE
102
WORD
Text scaling flag (0=no scale).
CHUP
104
WORD
Character rotation angle in tenths of degrees (supported only in 90 degree increments).
TEXTFG
106
WORD
Text foreground color.
SCRTCHP
108
LONG
Pointer to two contiguous scratch buffers used in creating text special effects.
SCRPT2
112
WORD
Offset from first buffer to second (in bytes).
TEXTBG
114
WORD
Text background color.
COPYTRAN
116
WORD
Copy raster mode (0=Opaque, 1=Transparent).
SEEDABORT
118
LONG
Pointer to a routine called by the seedfill routine at each line. If not needed during a seed fill you should point it to a routine like the following: 
seedabort:
  sub.l d0,d0
  rts

Line-A Font Headers

Raster system and GDOS fonts are linked to form a list of font headers which contain the information needed to render text. Outline text generated by FSM is inaccessible in this manner.

Each monospaced font contains a font header, character and horizonatal offset table, and font form. All data types are in "Little Endian" (Intel format) and as such must be byte-swapped before use.

The font form is a raster form with each character laid side-by-side on the horizontal plane. The first character is WORD aligned but padding within the form only occurs at the end of a scanline to force the next scanline to be WORD aligned.

Each font header contains a pointer to the next font in the list. The list is terminated by a NULL pointer. The font header format is as follows:

Name
Offset
Type Contents
font_id
0
WORD Font ID number (must be unique).
point
2
WORD Point size of font.
name
4
32 BYTEs ASCII Name of font.
first_ade
36
UWORD First ASCII character in font.
last_ade
38
UWORD Last ASCII character in font.
top
40
UWORD Distance from the top line of the font to the baseline.
ascent
42
UWORD Distance from the ascent line of the font to the baseline.
half
44
UWORD Distance from the half line of the font to the baseline.
descent
46
UWORD Distance from the descent line of the font to the baseline.
bottom
48
UWORD Distance from the bottom line of the font to the baseline.
max_char_width
50
UWORD Width of the widest character in the font.
max_cell_width
52
UWORD Width of the widest character cell in the font.
left_offset
54
UWORD Amount character slants left when skewed.
right_offset
56
UWORD Amount character slants right when skewed.
thicken
58
UWORD Number of pixels to smear for thickening.
ul_size
60
UWORD Size of an appropriate underline for the font.
lighten
62
UWORD Mask for character lightening.
skew
64
UWORD Mask for character skewing.
flags
66
UWORD Font type flags.
hor_table
68
LONG Pointer to the horizontal offset table. The horizontal offset table is an array of bytes with one entry per character denoting the pixel offset to the character.
off_table
72
LONG Pointer to the character offset table. The character offset table is an array of WORDs with one entry per character denoting the byte offset into the font form of the character.
dat_table
76
LONG Pointer to the character data.
form_width
80
UWORD Width of the font form in bytes.
form_height
82
UWORD Height of the font form in pixels.
next_font
84
LONG Pointer to the next font in the list (0=no more fonts).
reserved
88
UWORD Reserved for future use.

Line-A Function Calling Procedure

Line-A functions are called by simply inserting the opcode into the instruction stream. For example, the 'Hide Mouse' function is called with the following assembly language instruction:
dc.w $A00A

Generally, the Line-A initialization function is called ($A000) and the address of the variable and/or font header tables are stored. Prior to each Line-A call variables are set as explained in the Line-A Function Reference and the function is then called. There is no method of error reporting available.

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