diff options
Diffstat (limited to 'libc/bcc/__ldivmod.c')
| -rw-r--r-- | libc/bcc/__ldivmod.c | 195 |
1 files changed, 195 insertions, 0 deletions
diff --git a/libc/bcc/__ldivmod.c b/libc/bcc/__ldivmod.c new file mode 100644 index 0000000..4cb6d77 --- /dev/null +++ b/libc/bcc/__ldivmod.c @@ -0,0 +1,195 @@ +/************************************************************************/ +/* This file contains the BCC compiler helper functions */ +/* Function ldivmod */ + +#ifdef __AS386_16__ +#asm + .text + .even + +| ldivmod.s - 32 over 32 to 32 bit division and remainder for 8086 + +| ldivmod( dividend bx:ax, divisor di:cx ) [ signed quot di:cx, rem bx:ax ] +| ludivmod( dividend bx:ax, divisor di:cx ) [ unsigned quot di:cx, rem bx:ax ] + +| dx is not preserved + + +| NB negatives are handled correctly, unlike by the processor +| divison by zero does not trap + + +| let dividend = a, divisor = b, quotient = q, remainder = r +| a = b * q + r mod 2^32 +| where: + +| if b = 0, q = 0 and r = a + +| otherwise, q and r are uniquely determined by the requirements: +| r has the same sign as b and absolute value smaller than that of b, i.e. +| if b > 0, then 0 <= r < b +| if b < 0, then 0 >= r > b +| (the absoulute value and its comparison depend on signed/unsigned) + +| the rule for the sign of r means that the quotient is truncated towards +| negative infinity in the usual case of a positive divisor + +| if the divisor is negative, the division is done by negating a and b, +| doing the division, then negating q and r + + + .globl ldivmod + +ldivmod: + mov dx,di ! sign byte of b in dh + mov dl,bh ! sign byte of a in dl + test di,di + jns set_asign + neg di + neg cx + sbb di,*0 +set_asign: + test bx,bx + jns got_signs ! leave r = a positive + neg bx + neg ax + sbb bx,*0 + j got_signs + + .globl ludivmod + .even + +ludivmod: + xor dx,dx ! both sign bytes 0 +got_signs: + push bp + push si + mov bp,sp + push di ! remember b + push cx +b0 = -4 +b16 = -2 + + test di,di + jne divlarge + test cx,cx + je divzero + cmp bx,cx + jae divlarge ! would overflow + xchg dx,bx ! a in dx:ax, signs in bx + div cx + xchg cx,ax ! q in di:cx, junk in ax + xchg ax,bx ! signs in ax, junk in bx + xchg ax,dx ! r in ax, signs back in dx + mov bx,di ! r in bx:ax + j zdivu1 + +divzero: ! return q = 0 and r = a + test dl,dl + jns return + j negr ! a initially minus, restore it + +divlarge: + push dx ! remember sign bytes + mov si,di ! w in si:dx, initially b from di:cx + mov dx,cx + xor cx,cx ! q in di:cx, initially 0 + mov di,cx + ! r in bx:ax, initially a + ! use di:cx rather than dx:cx in order to + ! have dx free for a byte pair later + cmp si,bx + jb loop1 + ja zdivu ! finished if b > r + cmp dx,ax + ja zdivu + +| rotate w (= b) to greatest dyadic multiple of b <= r + +loop1: + shl dx,*1 ! w = 2*w + rcl si,*1 + jc loop1_exit ! w was > r counting overflow (unsigned) + cmp si,bx ! while w <= r (unsigned) + jb loop1 + ja loop1_exit + cmp dx,ax + jbe loop1 ! else exit with carry clear for rcr +loop1_exit: + rcr si,*1 + rcr dx,*1 +loop2: + shl cx,*1 ! q = 2*q + rcl di,*1 + cmp si,bx ! if w <= r + jb loop2_over + ja loop2_test + cmp dx,ax + ja loop2_test +loop2_over: + add cx,*1 ! q++ + adc di,*0 + sub ax,dx ! r = r-w + sbb bx,si +loop2_test: + shr si,*1 ! w = w/2 + rcr dx,*1 + cmp si,b16[bp] ! while w >= b + ja loop2 + jb zdivu + cmp dx,b0[bp] + jae loop2 + +zdivu: + pop dx ! sign bytes +zdivu1: + test dh,dh + js zbminus + test dl,dl + jns return ! else a initially minus, b plus + mov dx,ax ! -a = b * q + r ==> a = b * (-q) + (-r) + or dx,bx + je negq ! use if r = 0 + sub ax,b0[bp] ! use a = b * (-1 - q) + (b - r) + sbb bx,b16[bp] + not cx ! q = -1 - q (same as complement) + not di +negr: + neg bx + neg ax + sbb bx,*0 +return: + mov sp,bp + pop si + pop bp + ret + + .even + +zbminus: + test dl,dl ! (-a) = (-b) * q + r ==> a = b * q + (-r) + js negr ! use if initial a was minus + mov dx,ax ! a = (-b) * q + r ==> a = b * (-q) + r + or dx,bx + je negq ! use if r = 0 + sub ax,b0[bp] ! use a = b * (-1 - q) + (b + r) (b is now -b) + sbb bx,b16[bp] + not cx + not di + mov sp,bp + pop si + pop bp + ret + + .even + +negq: + neg di + neg cx + sbb di,*0 + mov sp,bp + pop si + pop bp + ret +#endasm +#endif |