[Coco] Setting graphics modes with Assembly

[Fedor Steeman]

Thanks, Robert, for the help, and that looks like a very elegant and useful
way to set the display offset.

I have dug up Don Inman's book on assembly language graphics and really
coming to terms with this. I keep on experimenting and learning...

Cheers,
Fedor


2008/12/20 Robert Gault <robert.gault at worldnet.att.net>

> The main "error" is the first JMP $ADFB. Since you want to return to your
> program not Basic, this should be JSR $ADFB. The second use of the ROM
> routine is OK because you do want to return to Basic and therefore should
> use JMP.
>
> $FFC0 is not The clear bit for the SAM rather A clear bit. All even bytes
> in that region act as clears, all odd bytes act as sets for a specific
> control toggle.
>
> While it can take more code, it would probably help you to use a generic
> routine to write to the SAM control register where you specify the actual
> control word rather than bytes.
>
> *Enter with regA equal to the 7-bit SAM value, regB = 7, regX=1st SAM byte
> *Leave with SAM set for some video offset
> * $E00/$200=7
>        LDA     #7      the above result
>        JSR     SETSAM
>        ...             Program code
> * Video offset controller for PMODEs
> SETSAM  LDB     #7      for 7-bit word
>        LDX     $FFC6   first SAM offset byte
> SAMLUP  RORA
>        BCC     SAM2
>        LEAX    1,X
>        STA     ,X+
>        BRA     SAM3
> SAM2    STA     ,X++
> SAM3    DECB
>        BNE     SAMLUP
>        RTS
>
> The above routine can be used to setup the SAM for any video location
> merely by changing the value in regA. The same type of routine can be used
> to set any SAM mode ($FFC0-$FFC5) just by loading regB with 3 and regX with
> $FFC0.
>
> Fedor Steeman wrote:
>
>> Hi all,
>>
>> I have been studying Chris Lomont's reference guide and, combined with
>> your
>> help, I am starting to come to grips with it. One thing I don't get yet,
>> though, is how to switch from one graphics mode to another. I tried simply
>> appending the following to the already written code, but that doesn't do
>> anything noticeable:
>>
>>
>>    LDA    #$E8        * load 1110 1000
>>    STA    VDGSET    * set pmode 3
>>
>> Also: how to set the equivalent of SCREEN 1,0 / 1,1 commands?
>>
>> This is my complete code so far:
>>
>> TXTSCR    EQU    $0400     *Start of text screen
>> HIRES1    EQU    $E00    *Start of hi-res graphics screen
>> VDGSET    EQU    $FF22        *PIA1 data port B: VDG Control output
>> V0CLR    EQU    $FFC0        *Clear bit for Sam Chip (Graphics mode)
>> V1SET    EQU    $FFC3        *Set V1 bit in Sam Chip (Graphics mode)
>> V2SET    EQU    $FFC5        *set V2 bit in Sam Chip (Graphics mode)
>> F0CLR    EQU    $FFC6        *Display Offset Binary, This is the CLR Bit
>> (Video page offset)
>> F0SET    EQU    $FFC7
>> F1SET    EQU    $FFC9
>> F2SET    EQU    $FFCB
>> POLCAT    EQU    $A000        *Color Basic rom poll keyboard routine
>>
>>    ORG    $7000
>> START    LDX    #HIRES1
>>    LDD    #$AAAA
>> LOOP1    STD     ,X++
>>    CMPX #HIRES1+$1800
>>    BCS     LOOP1
>> GMODE    STA    V0CLR    * set graphics mode
>>    STA    V1SET    * set graphics mode
>>    STA    V2SET    * set graphics mode
>>    LDA    #$F8    * load 1111 1000
>>    STA    F0SET    * set graphics mode
>>    STA    F1SET    * set graphics mode
>>    STA    F2SET    * set graphics mode
>>    STA    VDGSET    * set pmode 4 equivalent
>>    JMP    $ADFB    * wait for keypress
>>    LDA #$E8    * load 1110 1000
>>    STA    VDGSET    * set pmode 3 equivalent?
>>    JMP    $ADFB
>>    END START
>>
>> Thanks for any input!
>>
>> Cheers,
>> Fedor
>>
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>>
>>
>
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