Added a 160x192x2 TGI (graphics) driver to the VIC-20 library.

The driver requires a special linker configuration: "vic20-tgi.cfg".
The VIC-20 computer needs at least 8K of expansion RAM!

"tgidemo.c" needed to be adjusted because the VIC-20's vertical (y) range is greater than its horizontal (x) range -- the opposite of most other platforms.  Also, the circle demo would jam on the VIC-20.

libsrc/vic20/libref.s

@@ -1,10 +1,14 @@
 ;
-; Oliver Schmidt, 2013-05-31
+; 2013-05-31, Oliver Schmidt
+; 2018-03-11, Sven Michael Klose
 ;
 
-        .export         joy_libref
         .export         em_libref
+        .export         joy_libref
+        .export         tgi_libref
+
         .import         _exit
 
-joy_libref      := _exit
 em_libref       := _exit
+joy_libref      := _exit
+tgi_libref      := _exit

libsrc/vic20/tgi_stat_stddrv.s

@@ -0,0 +1,8 @@
+;
+; Address of the static standard TGI driver
+;
+; const void tgi_static_stddrv[];
+;
+
+        .import _vic20_hi_tgi
+        .export _tgi_static_stddrv := _vic20_hi_tgi

libsrc/vic20/tgi_stddrv.s

@@ -0,0 +1,13 @@
+;
+; Name of the standard TGI driver
+;
+; 2018-03-11, Sven Michael Klose
+;
+; const char tgi_stddrv[];
+;
+
+        .export _tgi_stddrv
+
+.rodata
+
+_tgi_stddrv:    .asciiz "vic20-hi.tgi"

libsrc/vic20/tgihdr.s

@@ -0,0 +1,67 @@
+;
+; This code sits immediately after the BASIC stub program.
+; Therefore, it's executed by that stub.
+;
+; 2018-04-17, Greg King
+;
+
+        .export         __TGIHDR__ : abs = 1    ; Mark as TGI housekeeper
+
+        .import         __MAIN_LAST__
+        .importzp       ptr1, ptr2, tmp1
+
+
+basic_reset     :=      $C000           ; vector to BASIC's cold-start code
+
+; This code moves the program to $2000.  That move allows $1000 - $1FFF
+; to be used by the TGI driver to hold its graphics data.
+
+.segment        "TGI1HDR"
+
+        lda     #<(tgi1end + prog_size) ; source
+        ldx     #>(tgi1end + prog_size)
+        sta     ptr1
+        stx     ptr1+1
+
+        lda     #<(tgi2hdr + prog_size) ; destination
+        ldx     #>(tgi2hdr + prog_size)
+        sta     ptr2
+        stx     ptr2+1
+
+        ldx     #<~prog_size
+        lda     #>~prog_size            ; use -(prog_size + 1)
+        sta     tmp1
+
+        ldy     #<$0000
+
+; Copy loop
+
+@L1:    inx                             ; bump counter's low byte
+        beq     @L4
+
+@L2:    tya                             ; will .Y underflow?
+        bne     @L3
+        dec     ptr1+1                  ; yes, do next lower page
+        dec     ptr2+1
+@L3:    dey
+
+        lda     (ptr1),y
+        sta     (ptr2),y
+        jmp     @L1
+
+@L4:    inc     tmp1                    ; bump counter's high byte
+        bne     @L2
+
+        jmp     tgi2hdr                 ; go to moved program
+tgi1end:
+
+.segment        "TGI2HDR"
+
+tgi2hdr:
+        jsr     tgi2end                 ; run actual program
+        jmp     (basic_reset)
+tgi2end:
+
+; The length of the TGI program (including the TGI2HDR segment)
+
+prog_size       =       __MAIN_LAST__ - tgi2hdr