Pl2 LINEA.DOC
CHAPTER NR°8:
THE LINEAS
- INTRODUCTION ***
- LineAs are graphical functions, they are very simple
but have the advantage of being really very fast. (Much more than the VDI...)
LineAs are commands of the 'BLITTER', i.e., it is the HARDWARE (Some PHYSICAL components of the computer and the routines that are executed by the PROCESSOR himself.) that execute these func- tions and not the SOFTWARE (The SOFTWARE part of your computer: ROM, programs...). If your ST is equipped with the BLITTER of course... (The MEGA 2ST and the MEGA 4ST are equipped.) NB: There is a SOFTWARE version of the BLITTER that is available in the public --- domain in the form of a desktop accessory... These functions are very fast since not coming from the SOFTWARE, the routines do not need to be interpreted by the PROCESSOR.
- To call the LineAs functions, you will have to provide the appro-
priate parameters in the VDI TABLE and also in an INTERNAL TABLE specific to LineAs. Some registers will also be used.
Indeed, the LineAs use the VDI's CONTROL, PTSIN, PTSOUT, INTIN, and INTOUT arrays. Registers d0, d1, d2, a0, a1, a2 will also be used, so you will have to be careful not to lose their content... Some parameters are also provided by the LineAs in return in d0, a0 or INTOUT (here again, be careful not to lose d0!)
We have seen that the LineAs use an INTERNAL TABLE:
It is in this table that the functions search for the parameters that we have deposited, the only problem is that the location of this internal table varies (hence its name).
To find the location of this internal table, just call the function $A000 with:
DC.W $A000
What a strange syntax isn't it??
The explanation is very simple: During the ASSEMBLY step of your listing, the ASCII codes representing the instructions and their operands will be translated into BINARY because the 68000 only recognizes information coded in binary: Thus, a NOP will be translated by %100111001110001, a RTS by %0100111 001110101 etc... and so for all instructions. If we translate these codes from binary to HEXADECIMAL, we obtain instructions coded in HEX: Thus, NOP will be written: $4E71 and RTS will be written: $4E75 etc...
However, -- You can REPLACE, in an ASSEMBLER program, an instruction by its HEX CODE equivalent if it is declared as such in the TEXT segment, it would suffice to write:
DC.W $4E71 instead of a NOP (or DC.B $4E, $71) or DC.W $4E75 instead of an RTS ... (or DC.B $4E, $75)
This remains valid for all the instructions of the 68000 (Even for labels).
But, ---- No 68000 instruction will be coded in HEX by a WORD of type:
$A... (or $F...)
We have therefore filled this lack (we rather took advantage of it!) by coding the LineAs functions in $A...
NB: There are also $F... routines but we will not use them
-- because they are not compatible between the OLD ROM and NEW ROM versions
of the ST.
It will therefore suffice to write DC.W $A... for the 68000 to decode this instruction by the LineA instruction of code $A...
There are a total of 16 commands that are installed and that can be translated this way, these are the 16 LineAs routines:
To call them simply DECLARE them in the TEXT segment:
The LineAs: ----------- $A000: Installs the internal table $A001: Places 1 point on the screen [...]
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