HPAVC/HPAVC CD-ROM.iso/ASMVLA00.ZIP/ASM2.TXT

Text File | 1993-03-21 | 6KB | 180 lines

─────────────────────────────────────────────────────────────────────────────
    ASM2.TXT - intro to keyboard and flow control
─────────────────────────────────────────────────────────────────────────────

    Alright.  This bit of code introduces control flow, keyboard input, and
    a way to easily print out one character.

────────
    First off, lets examine the easiest one: printing a character.

    It's like this: you put the character to print in DL, put 2 in AH and
    call interrupt 21h.  Damn easy.

────────
    Ok, lets look at the next easiest one: keyboard input.

    There are quite a few functions related to INT 16h (the keyboard
    interrupt.)  They are:

FUNCTION#
---------

    0h  -Gets a key from the keyboard buffer.  If there isn't one, it waits
            until there is.
            Returns the SCAN code in ah, and the ASCII translation in AL

    1h  -Checks to see if a key is ready to grab.  Sets the zero flag if a
            key is ready to grab.  Grab it with Fn# 0
            This also returns the same info about the key as Fn#0, but does
            not remove it from the buffer.

    2h  -Returns the shift flags in al.  They are:
            bit 7 - Insert active
            bit 6 - Caps lock active
            bit 5 - Num Lock active
            bit 4 - Scroll lock active
            bit 3 - Alt pressed
            bit 2 - Ctrl pressed
            bit 1 - Left shift pressed
            bit 0 - right shift pressed

    3h  -You can set the Typematic Rate and delay with this function
            registers must be set as follows
            AL = 5
            BH = Delay value (0-3: 250,500,750,1000 millisec)
            BL = Typematic rate (0-1fh) 1fh = slowest (2 chars per sec)
                0 =fastest (30 chars per second)

    4h  -Key Click control - not important

    5h  -STUFF the keyboard
        input:
            CH = scan code
            CL = ascii code

        output:
            al = 0 no error
            al = 1 keyboard buffer is full

   10h  -Same as #0, but its for the extended keyboard.  Checks all the keys.

   11h  -Same as #1, but for the extended keyboard.

   12h  -Same as #2, but AH contains additional shift flags:
            bit 7 - Sys req pressed
            bit 6 - Caps lock active
            bit 5 - Num Lock active
            bit 4 - Scroll lock active
            bit 3 - Right Alt active
            bit 2 - Right Ctrl active
            bit 1 - Left Alt active
            bit 0 - Right Alt active
        Al is EXACTLY the same as in Fn#2


WHERE AH= the function number when you call INT 16h

────────
    That's neat-o, eh?  Now on to flow controll via CMP and Jcc...

CMP:
───
    CMP is the same as SUB, but it does NOT alter any registers, only the
    flags.  This is used in conjunction with Jcc.

Jcc:
───
    Ok, Jcc is not a real instruction, it means 'jump if conditionis met.'

    I'll break this into 3 sections, comparing signed numbers, comparing
    unsigned numbers, and misc.

    Note that a number being 'unsigned' or 'signed' only depends on how you
    treat it.  That's why there are different Jcc for each...

    If you treat it as a signed number, the highest bit denotes whether it's
    negative or not.

    Prove to yourself that 0FFFFh is actually -1 by adding 1 to 0FFFFh.  You
    should get a big zero: 00000h.  (Remember that the number is ONLY 16 bits
    and the carry dissapears..)

UNSIGNED:
────────
    JA  -jumps if the first number was above the second number
    JAE -same as above, but will also jump if they are equal

    JB  -jumps if the first number was below the second
    JBE -duh...

    JNA -jumps if the first number was NOT above... (same as JBE)
    JNAE-jumps if the first number was NOT above or the same as..
            (same as JB)
    JNB -jumps if the first number was NOT below... (same as JAE)
    JNBE-jumps if the first number was NOT below or the same as..
            (same as JA)
    JZ  -jumps if the two numbers were equal (zero flag = 1)
    JE  -same as JZ, just a different name

    JNZ -pretty obvious, I hope...
    JNE -same as above...

SIGNED:
──────
    JG  -jumps if the first number was > the second number
    JGE -same as above, but will also jump if they are equal

    JL  -jumps if the first number was < the second
    JLE -duh...

    JNG -jumps if the first number was NOT >... (same as JLE)
    JNGE-jumps if the first number was NOT >=.. (same as JL)

    JNL -jumps if the first number was NOT <... (same as JGE)
    JNLE-jumps if the first number was NOT <=... (same as JG)

    JZ, JE, JNZ, JNE - Same as for Unsigned

MISC:
────
    JC  -jumps if the carry flag is set
    JNC -Go figgure...

    Here's the rest of them... I've never had to use these, though...

    JO  -jump if overflow flag is set
    JNO -...

    JP  -jump is parity flag is set
    JNP -...
    JPE -jump if parity even (same as JP)
    JPO -jump if parity odd (same as JNP)

    JS  -jumps if sign flag is set
    JNS -...


Here's the flags really quickly:
────────────────────────────────
bit#    8 7 6 5 4 3 2 1 0
        ─────────────────
symbol: O D I T S Z A P C

    O = OverFlow flag
    D = Direction flag  *
    I = Interrupt flag
    T = Trap flag
    S = Sign flag
    Z = Zero flag       *
    A = Auxiliary flag
    C = Carry flag      *

The * denotes the ones that you should know.

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That's it for now...  Until next time...

Draeden\VLA