1. SiS 315 An introductory exploration of features of the SVGA graphics processor used in our classroom’s workstations

2. Some SVGA examples • Our GPU is manufactured by SiS • Official programming info is ‘unavailable’ • Can find some “unofficial” info on internet • We explore some useful SiS 315 features • But remember: other GPU chips are NOT hardware-compatible (so our software is unlikely to work on other SVGA systems) • We sacrifice the advantage of “portability”

3. SiS Extended Registers • The typical Super VGA chip implements a number of “extra” hardware registers • Most often these appear as additions to the standard set of VGA registers established by IBM’s engineers: • Graphics Controller (9 registers) • Timer-Sequencer (5 registers) • CRT Controller (25 registers) • Attribute Controller (21 registers) • Our SiS graphics processor implements “extra” Sequencer registers (ports 0x3C4/0x3C5) and also “extra” CRTC registers (0x3D4/0x3D5)

4. SiS Sequencer registers • Five standard VGA Sequencer-registers: 0: the Reset register 1: the Clocking Mode register 2: the Map Mask register 3: the Character Map Select register 4: the Memory Mode register • SiS implements registers 5, 6, 7, … , etc.:

5. In-Class Exercise • Investigate the functionality of register 5 • Two-step process to read this register: outb( 5, 0x3C4 ); int val = inb( 0x3C5 ); • Two-step process to write this register: outb( 5, 0x3C4 ); outb( val, 0x3C5 ); • Alternatively: outw( (val<<8)|5, 0x3C4 );

6. Write a demo program • Remember to call function: iopl( 3 ); • Then create a program-loop • For every possible byte-value: • write that byte-value to Seq-register 5 • read the byte-value in Seq-register 5 • print out the value that was read back • What do you learn from this exercise?

7. Some further experiments • Repeat the previous experiment, but add a new step within your program-loop: 4) read (and print) Seq-register 6 • What do you learn from THIS exercise? • OK, try it again, but with Seq-register 7 in place of Seq-register 6 (Caution: Don’t try to ‘write’ new values into registers 6 or 7)

8. Detecting screen-height • The screen-height parameter is 11-bits • Lowest 8-bits are in CRTC register 0x12 • 9th and 10th bits are in CRTC register 0x07 • This much is standard on VGA hardware • 11th bit is in SiS Sequencer register 0x0A • We need to read all three registers, then extract and assemble the various bits

9. The screen-height bits crtc-0x12 (Vert Displ Enable End) crtc-0x07 (Overflow Register) screen_height - 1 ? seq-0x0A

10. Detecting screen-width • The screen-width parameter is 10 bits • It measures the width in 8-pixel units • Lowest 8-bits are in CRTC register 0x01 • 9th and 10th bits in Ext’d Seq register 0x0B • The parameter-value is actually one less • It gives the id-number of the final unit • So we increment it, then multiply by 8

11. The screen-width bits crtc-0x01 (Horizontal Display End) screen_width – 1 (in 8-pixel units) seq-0x0B

12. Detecting CRT start-address • Very important parameter for animation! • This parameter is stored as 26 bits • It measures start-address in 4-byte units • Lowest byte stored in crtc register 0x0D • Next lowest byte is in crtc register 0x0C • Sequencer register 0x0D holds next byte • Sequencer register 0x37 has 2 more bits

13. The crt start-address bits crtc-0x0C (Start Addr Hi) crtc-0x0D (Start Addr Lo) crt start-address (in 4-byte units) seq-0x0D seq-0x37

14. Detecting CRT Line-Compare • CRTC-address resets when counter == LC • This is useful for certain “special effects” • Screen is split into two separate “windows” • Lower window shows VRAM address 0 • Upper window is from crt start-address • Lowest 8-bits from CRT register 0x18 • 9th bit from CRT register 0x07 • 10th bit from CRT register 0x09 • More bits from Sequencer register 0x0F

15. The Line-Compare bits crtc-0x18 (Line Compare register) crtc-0x07 (Overflow register) Line-Compare crtc-0x09 (Max Scan Line) seq-0x0F

16. In-Class Experiment • Reprogram the ‘Line Compare’ parameter with a value that is equal to one-half of the current screen-height • What visual effect do you observe? • Do you see a similar effect under Linux in both ‘text mode’ and in graphics modes?