[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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