Difference between revisions of "Programming:Overscan"
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Revision as of 16:47, 12 March 2007
From the The Unofficial Amstrad WWW Resource
;; This example shows a simple method for a overscan ;; screen. ;; ;; This example will work on CPC, CPC+ and KC Compact. ;; ;; This example uses the 6845 CRTC, and shows how ;; 32k of pixel data can be displayed by the CRTC. ;; This example is compatible with CRTC type 2. (MC6845) ;; ;; the screen is 48x35 visible characters in dimension, ;; and each character is 8 scanlines tall. ;; the visible window is moved so that the display ;; will fill the entire monitor screen. ;; The screen base address is initialised, so that ;; MA11=MA10=1, and the internal MA counter of the CRTC will ;; change MA12 during the display, and cause the CRTC to display ;; data from a 32k range. The actual visible display uses approx ;; 24k of RAM. After this setup, the screen does not need to ;; re-initialised for the overscan to be maintained. ;; ;; the screen is setup for a PAL display. ;; ;; This example will compile with the MAXAM assembler ;; or the built-in assembler of WinAPE32. ;; ;; Kevin Thacker 2002 ;; the origin of this source code is not important org &4000 ;; initialise a display window of 48x35 characters ;; each character is 8 scanlines tall ;; set character height to 8 scanlines ld bc,&bc09 out (c),c ld bc,&bd00+7 out (c),c ;; set default horizontal total (set horizontal total to 64 CRTC characters ;; = 64 microseconds) ld bc,&bc00 out (c),c ld bc,&bd00+&3f out (c),c ;; set default vertical total (set vertical total to 39 CRTC character-lines) ld bc,&bc04 out (c),c ld bc,&bd00+38 out (c),c ;; setup default horizontal and vertical sync widths ld bc,&bc03 out (c),c ld bc,&bd00+&8e out (c),c ;; setup default vertical adjust ld bc,&bc05 out (c),c ld bc,&bd00 out (c),c ;; setup default interlace & skew ld bc,&bc08 out (c),c ld bc,&bd00 out (c),c ;; set width of display window ld bc,&bc01 out (c),c ld bc,&bd00+48 out (c),c ;; set horizontal sync position; and therefore the ;; horizontal position of the display window ;; within the monitor display ld bc,&bc02 out (c),c ld bc,&bd00+48 out (c),c ;; set height of display window ld bc,&bc06 out (c),c ld bc,&bd00+35 out (c),c ;; set vertical sync position; and therefore the ;; vertical position of the display window ;; within the monitor display ld bc,&bc07 out (c),c ld bc,&bd00+35 out (c),c ;; set display start ;; force MA11=MA10=1, so that the internal MA ;; counter will increment enough to change MA12. ;; the displayed data is &0000-&7fff ld bc,&bc0c out (c),c ld bc,&bd00+&0c out (c),c ld bc,&bc0d out (c),c ld bc,&bd00+0 out (c),c ret