I am writing a program that calculate arctan function.
Function that does all work works fine, but when i try get bcd-value fbstp command store weird value and fstp store right value.
segment data
result: resb 10
tmp_val: dd 000000000h
precs: dq 03a83126fh ; 0.001
arg: dd 03f800000h ; 0.7
segment code
..start:
mov ax,data
mov ds, ax
finit
call calc_value
fst dword[tmp_val]
fbstp tword [result]
mov ax, 4C00h
int 21h
And after execution in memory
at tmp_val: 0x3f490edb (0.785383)
and at result: 0x0000000000000000001
And i write this for DOS
The intel instruction set reference says this for fbstp:
If the source value is a non-integral value, it is rounded to an integer value, according to rounding mode specified by the RC field of the FPU control word.
As such, a result of 1 sounds right.