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in the last chapter we talked about fixed points and their limitations (assembly has integers you just have to give them neg value) of shr,shl,ror,rol and the need to detect zero so what we have is flags, 16 bit register the each bit is a different status of something that happed.
major flags : CF - Carry flag is set when there is a borrow in some command lets say "sub ax,13" and ax is 21 so a borrow will apear. PF - Parity flag is set when the LSB of the operation is even, lets say "add ax,11" and ax is 11 so the flag will be set (even) ZF - Zero flag is set when the operation is zero lets say "dec ax" and ax is now 0 so ZF will be set. OF - Overflow flag is set when the operation is beyond the limitation of the register lets say "add ax,65535" (ax=100) so the flag will be set. there are more flags but much of them aren't used so (if you want to know open a book or norton guides to assembler) now before anything else a most important command is : CMP register,value - Compare a register to a certain value and set all needed flags. lets say "cmp ax,0" and ax is 0 so the ZF will be set. but (you may ask) how do we use those flags well the answer is conitional jumps !! JNE - Jump if not equal JZ - Jump if zero JE - Jump if equal JB - Jump if below JA - Jump if above JAE - Jump if above and equal JBE - Jump if below and equal JC - Jump if carry JNC - Jump if not carrt JS - Jump if sign JNS - Jump if not sign JMP - Unconditional jump note : all of these conditional jumps (not jmp) can jump only in the range of near (256/0ffh bytes) and only jmp can jump to the entire code !!!
use - JMP label example : mov ax,10 mov cx,20 mov dx,0 div cx ; does 10/20 result is 0 (if you don't know that so go back ; and repeat the commands. )
cmp ax,0 ; is ax 0 jz @iszero ; then it is so jump to label mov cx,ax mov dx,0 div cx ; it will never reach it cause it will never be larger then ; zero (if you comment the jz so you will get divide by 0) @iszero : ; this is a label the "@" before the label is optional mov ax,4c00h ; terminate int 21h now lets get back to our fixed points that we want to convert : mov ax,100 ; our circle radios mov cx,181 ; 181/256 is 0.707 (sin(45)) mul cx ; multiply (result : 18,100) cmp ax,0 ; is ax 0 jb @neg1 ; jmp below so if neg jump to label shr ax,8 jmp @done ; don't want to get with the div @neg1 : mov dx,0 mov cx,256 idiv cx ; ax/256 just use idiv it's a div for integers @done : mov ax,4c000h ; terminate int 21h so that's about it in jumps and flags, I sugest that you will train a bit about all the stuff that you have learned in those chapters cause it is very important to understand those (those are the needed basic) in the next chpaters we will start transfering memory blocks, and use graphics.
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LESSON5 - FLAGS AND JUMPS |