nasm - ASM question, two's complement -

-

so book "assembly language step step" awesome, sort of cryptic how two's complement works when working on actual memory , register data. along that, i'm not sure how signed values represented in memory either, feel might what's keeping me confused. anywho...

it says: "-1 = $ff, -2 = $fe , on". understand two's complement of number multiplied -1 , when added original give 0. so, ff hex equivalent of 11111111 in binary, , 255 in decimal. question is: what's book saying when says "-1 = $ff"? mean -255 + -1 give 0 also, didn't explicitly, set of flag?

so in practice... let's have 11h, 17 in decimal, , 00100001 in binary. , value in al. neg al, , set cf , sf, , change value in al to... 239 in decimal, 11101111 in binary, or efh? don't see how 17 * -1? or poorly worded explanation book, means gives value need cause overflow?

thanks!

in two's complement, bytes, (-x) == (256 - x) == (~x + 1). (~ c'ish not operator, flips bits in operand.)

let's have 11h.

100h - 11h == efh (256 - 17  == 239)

note, 256 works bytes, cause they're 8 bits in size. 16-bit words you'd use 2^16 (65536), dwords 2^32. note math mod 256 bytes, 65536 shorts, etc.

or, using not/+1,

~11h = eeh +1...  efh

this method works words of sizes.


Comments

Post a Comment

Popular posts from this blog

php - What is the difference between $_SERVER['PATH_INFO'] and $_SERVER['ORIG_PATH_INFO']? -

-
what’s difference between $_server['path_info'] , $_server['orig_path_info'] ? how use them? when run print_r($_server) , path_info , orig_path_info not present in array. why not? how can enable them? i have read php manual on them, still don’t understand them. the path_info variable present if invoke php script this: http://www.example.com/phpinfo.php/hello_there it's /hello_there part after .php script. if don't invoke url that, there won't $_server["path_info"] environment variable. the porig_ prefix uncommon. path_info standard cgi-environment variable, , should never prefixed. did read that? (there issues around php3/php4 if invoked php interpreter via cgi-bin/ - hardly has such setups today.) for reference: http://www.ietf.org/rfc/rfc3875
Read more

fortran - Function return type mismatch -

-
i'm attempting recode old c++ program in fortran make use of lapack (i'm aware c++ have lapack++, i'm having lot of trouble installing it, gave up). i didn't have problems compilation, when had variables declared real . when started coding section of program required lapack, found parameters passed dsyev need double precision . tried change double precision (including changing hard coded numbers double precision counterparts, i.e. 0.0 -> 0.0d0) when try compile, following error of functions , subroutines i've written: error: return type mismatch of function <function> @ (1) (real(4)/real(8)) i'm not sure coming from, in program has been changed double precision. for example, i've declared following: double precision :: alpha(3),d(3),zeta1,zeta2 double precision :: a1(3),a2(3),d1(3),d2(3) double precision :: pi pi = 3.14159265359d0 alpha = (/0.109818d0, 0.405771d0, 2.22766d0/) d = (/0.444635d0, 0.535328d0, 0.154329d0 /) 10 i=1,3 ...
Read more

queue - mq_receive: message too long -

-
i implementing communication between 2 processes using queue. problem when call function mq_receive, error: message long. i have done following: struct mq_attr attr; long size = attr.mq_msgsize; .... // initializing queue "/gateway" int rc = mq_receive(gateway, buffer, size, &prio); if print size value, size=1, while when print same size program (got same mechanism), not long integer ( -1217186280 )... how can solve error?....so while size = 1, believe it's right "message long" why 1? p.s. have tried put : int rc = mq_receive(gateway, buffer, sizeof(buffer), &prio); but no result. it seems need read docs more carefully. when call mq_receive should pass size of destination buffer. this size must greater mq_msgsize attribute of queue . in addition, seems have error in queue attributes initialisation makes proper mq_receive call impossible. here standard message queue session: fill mq_attr struct ( doc ): struct mq_a...
Read more