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The CRTC Interface

The CRTC Interface

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The CRTC Interface

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  1. The CRTC Interface Essential aspects of display page and cursor control for standard 80-column by 25-row text

  2. Problem background • For proper display of text that applications wish to write to the STDOUT device-file, our ‘os630’ program will need to reposition the CRT cursor • This will involve directly programming certain registers in the CRT Controller (i.e., hardware) • For cooperation with messages that our ‘trackldr’ boot-loader displays (it uses int-0x10), we also need to set some variable-values located in the ROM-BIOS DATA AREA (i.e., software) • This lesson will explore these two issues

  3. Hardware registers • The CRT Controller is a peripheral chip • It implements 25 standard CRTC registers • Access to these registers is accomplished via a multiplexing scheme that uses just two I/O port-addresses: address-port: 0x03D4 data-port: 0x03D5

  4. The relevant CRTC registers • We are only concerned with 6 (of the 25) standard CRTC registers: • 0x0A: CURSOR_START • 0x0B: CURSOR_END • 0x0C: START_ADDRESS_HI • 0x0D: START_ADDRESS_LO • 0x0E: CURSOR_LOCATION_HI • 0x0F: CURSOR_LOCATION_LO

  5. 8x16 character box scanline 0 scanline 1 scanline 2 scanline 3 scanline 4 scanline 5 scanline 6 scanline 7 scanline 8 scanline 9 scanline 10 scanline 11 Cursor_start = 12 scanline 12 scanline 13 Cursor_end = 13 scanline 14 scanline 15

  6. CURSOR START/END 7 6 5 4 3 2 1 0 dis- able starting_scanline Index 0x0A: CURSOR_START REGISTER 7 6 5 4 3 2 1 0 skew ending_scanline Index 0x0B: CURSOR_END REGISTER

  7. Organization of VRAM Page 7 4KB Page 6 4KB Page 5 4KB Page 4 4KB Page 3 4KB Page 2 4KB Page 1 4KB Page 0 4KB Base_Address = 0xB8000

  8. Changing the visual page 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 START_ADDRESS_HI START_ADDRESS_LO register-index 0x0D register-index 0x0C Programming example: // switches display to vram page 5 // the offset to visual page 5 (in words) is 0x2800 (= 5 * 2048) mov dx, #0x03D4 // port-address in register DX mov ax, #0x280C // value=0x28, register=0x0C out dx, ax // write value to CRTC register mov ax, #0x000D // value=0x00, register=0x0D out dx, ax // write value to CRTC register

  9. Moving the CRT’s cursor 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 CURSOR_LOCATION_HI CURSOR_LOCATION_LO register-index 0x0E register-index 0x0F Programming example: // moves cursor to row 4, column 9, on page 0 mov dx, #0x03D4 // port-address in register DX mov bx, #4 // row-number imul bx, #80 // times cells-per-row add bx, #9 // plus column-number mov ah, bh // offset’s MSB mov al, #0x0E // CURSOR_HI index out dx, ax // write value to CRTC register mov ah, bl // offset’s LSB mov al, #0x0F // CURSOR_LO index out dx, ax // write value to CRTC register

  10. Scrolling the screen • Here’s a code-fragment that will scroll the contents of vram page 0 up by one line: mov ax, 0xB800 // address vram page 0 mov ds, ax // with DS register mov es, ax // also ES register mov di, #0 // destination is top line mov si, #160 // source is one line lower cld // do forward copying mov cx, #1920 // 24 times 80 rep movsw mov ax, #0x0720 // blank character/color mov cx, #80 // characters on bottom line rep stosw

  11. Linux uses hardware scrolling • The value of the CRT START_ADDRESS is reprogrammed, to change the region of visible vram by one line (i.e., add #80) • So instead of subdividing vram into eight 4KB pages, the entire 32KB vram is one continuous page, but only partially visible • To scroll up by one line, Linux adds #80 to the value of the CRT_START_ADDRESS

  12. The ROM-BIOS variables • Several variables in the ROM-BIOS DATA AREA are used by the VIDEO ROM-BIOS routins (i.e., int-0x10) to keep track of the current visisible page and of the positions of the cursors on each of the eight pages • The locations of these variables are part of the IBM-PC BIOS standard, and as such are widely documented

  13. Standard addresses 0x449 (byte) video mode-number 0x44A (word) number of columns on screen 0x44C (word) current page-size (in bytes) 0x44E (word) current page-address 0x450 (byte array) cursor-positions (col,row) 0x460 (cursor type) (END, START) 0x462 (byte) current page-number 0x463 (word) CRTC i/o port-address

  14. Real-mode INT-0x10 services • 0x00: set_display_mode • 0x01: set_cursor_type • 0x02: set_cursor_position • 0x03: get_cursor_position_and_type • 0x05: select_new_video_page • 0x06: scroll_current_page_up • 0x07: scroll_current_page_down • 0x08: read_char_and_attrib_from_screen • 0x09: write_char_and_attrib_to_screen • 0x0A: write char_only_to_screen • 0x0E: write_teletype_to_active_page • 0x0F: return_video_status • 0x13: write_string NOTE: These ROM-BIOS services are not available in protected-mode

  15. Cursor-movement demo • To illustrate reprogramming of the six CRT controller registers, we wrote ‘arrows.s’ • It lets the user control the cursor position and visible page by using arrow-keys • It also changes the height of the cursor • An unusual feature (not recommended) is its use of “polled mode” keyboard device-programming (instead of “interrupt-driven”)

  16. Polling the status-register • The keyboard interrupts are masked • The keyboard controller’s status-register is read and reread in a tight loop until bit #0 is set, indicating that a key was pressed and thus the output-buffer is now “full” • Then the output-buffer register is read by the CPU to get the key’s “scancode” • For arrow-keys, the cursor is moved • Other keys are ignored (except ESCAPE)

  17. Disadvantage of polling • Almost all of the CPU’s time is consumed by continually reading the status-register • So this would not be a good design to use in writing a multitasking operating system • On the other hand, for single-tasking it has the advantage of not requiring a interrupt service routine to be written, so the demo code can be shorter and simpler

  18. Another noteworthy feature • The ‘arrows.s’ demo uses a ‘jump-table’ to efficiently dispatch control to appropriate subroutines, based on a variable’s value • This is a similar programming situation to using a ‘switch’ statement in C/C++ • The jump-table avoids the long chain of ‘compare-and-branch’ statements for all the various possible cases that can occur

  19. In-Class exercise • To insure your mastery of the jump-table concept, and cement your grasp of how the CRTC START_ADDRESS registers are programmed, try modifying the demo to incorporate these additional actions: when one of the function-keys F1-F7 is pressed, the display is switched to the correspondingly numbered display-page