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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:


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..)


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...


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


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.
That's it for now... Until next time... Draeden\VLA


; ASM2.ASM ; Prints messages get keyboard input and has control flow DOSSEG .MODEL SMALL .STACK 200h .DATA Prompt db 13,10,"Do you want to be prompted again? (Y/N) $" NoMessage db 13,10,"Ok, then I won't prompt you anymore.$" YesMessage db 13,10,"Here comes another prompt!$" UnKnownKey db 13,10,"Please hit either Y or N.$" .CODE START: mov ax,@DATA ;moves the segment of data into ax mov ds,ax MainLoop: mov ah,9 mov dx,offset Prompt int 21h ;print a message mov ah,0 int 16h ;get a key, returned in AX ;AL is the ASCII part ;AH is the SCAN CODE push ax mov dl,al mov ah,2 int 21h ;print character in dl pop ax cmp al,"Y" ;was the character a 'Y'? jne NotYes ;nope it was Not Equal mov ah,9 mov dx,offset YesMessage int 21h jmp MainLoop NotYes: cmp al,"N" ;was the character a 'N' je ByeBye ;Yes, it was Equal mov dx,offset UnknownKey mov ah,9 int 21h jmp MainLoop ByeBye: mov dx,offset NoMessage mov ah,9 int 21h mov ax,4c00h ;Returns control to DOS int 21h ;MUST be here! Program will crash without it! END START