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stack, how does it work ? the law is last in first out, the principle is if I enter a series of numbers like : 1 34 54 54 65 and then I take them out I will get : 65 54 54 34 1. in assembly the command is push (it can push only 16bit) and the reverse is pop. so lets say I want to save the ds and later get it back :
push ds . . . . pop ds mov ax,4c00h ; short cut for : mov ah,4ch mov al,0 int 21h
ok now you've got the basic of assembly (I hope). in order to pass to the next level we have to explore the structure of the 8 bit register !!!
now bit is a condition that can be 0 or 1 so if we have 8 bit so the number of combinations can be 2^8 which is 256 (0..255) the limit of one byte (or word 2^16 65535) so assembly gives us the tools to modify and alter those bits.
SHR destination,count : Shift arithmic right divide the number by count^2 so 1 is 2 and 2 is 4 and so on "shr ax,4" will divide ax by 16 (4^2) SHL destination,count : Shift arithmic left multiply the number by count^2 so 1 is 2 and 2 is 4 and so on "shr ax,4" will multiply ax by 16 (4^2) ROR destination,count : Rotate right rotates the bits right so bit 0 is now bit 7 (16 bit, bit 0 is bit 15) and does is by count.
"ror al,8" will not affect cause there are only 8 bits so a whole round will be preformed. ROL destination,count : Rotate left rotates the bits right so bit 7 is now bit 0 (16 bit, bit 15 is bit 0) and does is by count. "ror al,8" will not affect cause there are only 8 bits so a whole round will be preformed. DIV factor : Divide will divide ax (or al) by the factor ("dx" must be set to 0), "div cl" will divide al by cl, "div cx" will divide ax by cx. MUL factor : Multiply will multiply ax (or al) by the factor (dx will be altered), if the operation is 8 bit "div cl" then the sum will be stored in ax (if it passes 8 bit) "mul cl" will mul ax by cl, "mul cx" will multiply ax by cx. NEG destination : Negation what it does he reverses negativity lets say we have 4 so it will become -4 and vise verse (n*-1)
as you might have noticed no floating points nor integers has been discussed well assembly has no floating points (don't be shocked) because you can still use the precision well how ?there are fixed points. fixed points are precision that was shifted before and after the calc been shifted back. fixed points example : mov ax,100 ; our circle radios mov cx,181 ; 181/256 is 0.707 (sin(45)) mul cx ; multiply (result : 18,100) shr ax,8 ; now the number is shifted back (n / 256 = shr n,8 cause ; 2^8 is 256) so the result is 70.7 (the point disapears) ; number is trunced to 70 (if you check 100*0.707 is 70.7) the fixed points are much faster then real (double) and usually the sin cos table is being written onto a file predifined by pascal,c example for extracting cos(56) to fix : var a:integer ; begin a:=trunc(cos(56*pi/180)*256) ; end ; now first we converted deg to rad and then multiply it by our factor. but, but, but noticed shr,shl,ror,rol do not work on integers, it will get wacked and will bug you program, so we need to detect if it's lower/higher then zero so we go to next chapter (flags and jumps) |
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LESSON4 - STACK (PUSH,POP) AND MORE COMMANDS |