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

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”

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)

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.:

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 );

Write a demo program Remember to call function: iopl( 3 ); Then create a program-loop For every possible byte-value: 1)write that byte-value to Seq-register 5 2)read the byte-value in Seq-register 5 3)print out the value that was read back What do you learn from this exercise?

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)

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

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

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 9 th and 10 th 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

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

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

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

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 9 th bit from CRT register 0x07 10 th bit from CRT register 0x09 More bits from Sequencer register 0x0F

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

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?