1	1. Example use of I/O instructions: boot loader
     2	
     3	    Below is a part of WeensyOS boot loader (for WeensyOS Schedos, which is part of
     4	    lab 4).
     5	
     6	    The main point for today’s class is that this code demonstrates I/O,
     7	    specifically with the disk: the bootloader reads in the kernel from the disk.
     8	
     9	    See the functions waitdisk() and readsect(). Compare to Figures 36.5
    10	    and 36.6 in OSTEP
    11	
    12	
    13	
    14	    #define SECTORSIZE 512
    15	
    16	    void waitdisk(void)
    17	    {
    18	        // wait for disk reaady
    19	        while ((inb(0x1F7) & 0xC0) != 0x40)
    20	            /* do nothing */;
    21	    }
    22	
    23	    void readsect(void *dst, uint32_t sect)
    24	    {
    25	        // wait for disk to be ready
    26	        waitdisk();
    27	
    28	        outb(0x1F2, 1); // count = 1
    29	        outb(0x1F3, sect);
    30	        outb(0x1F4, sect >> 8);
    31	        outb(0x1F5, sect >> 16);
    32	        outb(0x1F6, (sect >> 24) | 0xE0);
    33	        outb(0x1F7, 0x20); // cmd 0x20 − read sectors
    34	
    35	        // wait for disk to be ready
    36	        waitdisk();
    37	
    38	        // read a sector
    39	        insl(0x1F0, dst, SECTORSIZE/4);
    40	    }
    41	
    42	
    43	2. Another example: reading keyboard input
    44	
    45	    The code below is an excerpt from WeensyOS’s x86.c.
    46	
    47	    This reads a character typed at the keyboard (which shows up on the
    48	    "keyboard data port" (KBDATAP)), and converts it to a digit. This
    49	    code is not called in lab4; it was called in lab1. (Use grep to
    50	    convince yourself of this!)
    51	
    52	    /* Excerpt from WeensyOS x86.c. Comments from kbd.h in xv6 */
    53	
    54	    #define KBSTATP 0x64 // keyboard controller status port (I)
    55	    #define KBS_DIB 0x01 // keyboard data in buffer
    56	    #define KBDATAP 0x60 // keyboard data port (I)
    57	    
    58	    int console_read_digit(void)
    59	    {
    60	        uint8_t data;
    61	
    62	        if ((inb(KBSTATP) & KBS_DIB) == 0)
    63	            return −1;
    64	        
    65	        data = inb(KBDATAP);
    66	
    67	        if (data >= 0x02 && data <= 0x0A)
    68	            return data − 0x02 + 1;
    69	        else if (data == 0x0B)
    70	            return 0;
    71	        else if (data >= 0x47 && data <= 0x49)
    72	            return data − 0x47 + 7;
    73	        else if (data >= 0x4B && data <= 0x4D)
    74	            return data − 0x4B + 4;
    75	        else if (data >= 0x4F && data <= 0x51)
    76	            return data − 0x4F + 1;
    77	        else if (data == 0x53)
    78	            return 0;
    79	        else
    80	            return −1;
    81	    }
    82	
    83	3.  Memory−mapped I/O
    84	
    85	    An example is the console printing code from WeensyOS. Here are
    86	    excerpts from lib.h and lib.c:
    87	
    88	    /* Compare the addresses below to the map above. */
    89	    #define CONSOLE_BEGIN ((uint16_t *) 0x000B8000)
    90	    #define CONSOLE_END (CONSOLE_BEGIN + 80 * 25)
    91	
    92	    /*
    93	    * prints a character to the console at the specified
    94	    * cursor position in the specified color.
    95	    */
    96	
    97	    static uint16_t *
    98	    console_putc(uint16_t *cursor, unsigned char c, int color)
    99	    {
   100	        if (cursor >= CONSOLE_END)
   101	            cursor = CONSOLE_BEGIN;
   102	
   103	        if (c == ’\n’) {
   104	            int pos = (cursor − CONSOLE_BEGIN) % 80;
   105	
   106	            /* what does this do? */
   107	            for (; pos != 80; pos++)
   108	                *cursor++ = ’ ’ | color;
   109	        } else
   110	            *cursor++ = c | color;
   111	
   112	        return cursor;
   113	    }