I am writing a program that calculates the arctan function.
The calc_value function works fine, but when I try to get the BCD value from fbstp, the instruction stores a weird value. fst on the other hand stores the 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
After execution in memory:
at tmp_val: 0x3F490EDB (0.785383)
at result: 0x00000000000000000001
ps: 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.