228 lines
6.4 KiB
NASM
228 lines
6.4 KiB
NASM
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;[]-----------------------------------------------------------------[]
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;| H_LDIV.ASM -- long division routine |
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;| C/C++ Run Time Library Version 4.0 |
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;| |
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;| Copyright (c) 1987, 1991 by Borland International Inc. |
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;| All Rights Reserved. |
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;[]-----------------------------------------------------------------[]
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.model medium
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INCLUDE RULES.ASI
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.386C ;JAB - we use 386 instructions
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_TEXT segment public byte 'CODE'
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assume cs:_TEXT
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public LDIV@
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public F_LDIV@
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public N_LDIV@
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public LUDIV@
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public F_LUDIV@
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public N_LUDIV@
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public LMOD@
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public F_LMOD@
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public N_LMOD@
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public LUMOD@
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public F_LUMOD@
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public N_LUMOD@
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N_LDIV@:
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pop cx ;fix up far return
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push cs
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push cx
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LDIV@:
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F_LDIV@:
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xor cx,cx ; signed divide
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jmp short common
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; JAB
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;
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; If we're using a 386 or better, the two instructions above get patched
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; to be NOP's (4 of them). So, instead of using the looping code,
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; we use the 386's long divide instruction.
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;
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; The stack after setting up the stack frame:
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; 12[bp]: divisor (high word)
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; 10[bp]: divisor (low word)
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; 8[bp]: dividend (high word)
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; 6[bp]: dividend (low word)
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; 4[bp]: return CS
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; 2[bp]: return IP
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; 0[bp]: previous BP
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;
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IDEAL
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push bp
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mov bp,sp ;Save BP, and set it equal to stack
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mov eax,[DWORD PTR bp+6]
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cdq
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idiv [DWORD PTR bp+10]
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mov edx,eax
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shr edx,16
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pop bp ;Restore BP
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retf 8 ;Return to original caller
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MASM
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N_LUDIV@:
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pop cx ;fix up far return
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push cs
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push cx
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LUDIV@:
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F_LUDIV@:
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mov cx,1 ; unsigned divide
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jmp short common
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N_LMOD@:
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pop cx ;fix up far return
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push cs
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push cx
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LMOD@:
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F_LMOD@:
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mov cx,2 ; signed remainder
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jmp short common
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N_LUMOD@:
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pop cx ;fix up far return
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push cs
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push cx
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LUMOD@:
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F_LUMOD@:
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mov cx,3 ; unsigned remainder
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;
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; di now contains a two bit control value. The low order
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; bit (test mask of 1) is on if the operation is unsigned,
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; signed otherwise. The next bit (test mask of 2) is on if
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; the operation returns the remainder, quotient otherwise.
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;
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common:
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push bp
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push si
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push di
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mov bp,sp ; set up frame
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mov di,cx
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;
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; dividend is pushed last, therefore the first in the args
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; divisor next.
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;
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mov ax,10[bp] ; get the first low word
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mov dx,12[bp] ; get the first high word
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mov bx,14[bp] ; get the second low word
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mov cx,16[bp] ; get the second high word
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or cx,cx
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jnz slow@ldiv ; both high words are zero
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or dx,dx
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jz quick@ldiv
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or bx,bx
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jz quick@ldiv ; if cx:bx == 0 force a zero divide
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; we don't expect this to actually
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; work
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slow@ldiv:
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test di,1 ; signed divide?
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jnz positive ; no: skip
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;
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; Signed division should be done. Convert negative
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; values to positive and do an unsigned division.
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; Store the sign value in the next higher bit of
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; di (test mask of 4). Thus when we are done, testing
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; that bit will determine the sign of the result.
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;
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or dx,dx ; test sign of dividend
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jns onepos
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neg dx
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neg ax
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sbb dx,0 ; negate dividend
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or di,0Ch
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onepos:
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or cx,cx ; test sign of divisor
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jns positive
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neg cx
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neg bx
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sbb cx,0 ; negate divisor
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xor di,4
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positive:
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mov bp,cx
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mov cx,32 ; shift counter
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push di ; save the flags
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;
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; Now the stack looks something like this:
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;
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; 16[bp]: divisor (high word)
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; 14[bp]: divisor (low word)
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; 12[bp]: dividend (high word)
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; 10[bp]: dividend (low word)
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; 8[bp]: return CS
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; 6[bp]: return IP
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; 4[bp]: previous BP
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; 2[bp]: previous SI
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; [bp]: previous DI
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; -2[bp]: control bits
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; 01 - Unsigned divide
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; 02 - Remainder wanted
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; 04 - Negative quotient
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; 08 - Negative remainder
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;
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xor di,di ; fake a 64 bit dividend
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xor si,si ;
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xloop:
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shl ax,1 ; shift dividend left one bit
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rcl dx,1
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rcl si,1
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rcl di,1
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cmp di,bp ; dividend larger?
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jb nosub
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ja subtract
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cmp si,bx ; maybe
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jb nosub
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subtract:
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sub si,bx
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sbb di,bp ; subtract the divisor
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inc ax ; build quotient
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nosub:
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loop xloop
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;
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; When done with the loop the four register value look like:
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;
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; | di | si | dx | ax |
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; | remainder | quotient |
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;
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pop bx ; get control bits
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test bx,2 ; remainder?
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jz usequo
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mov ax,si
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mov dx,di ; use remainder
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shr bx,1 ; shift in the remainder sign bit
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usequo:
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test bx,4 ; needs negative
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jz finish
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neg dx
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neg ax
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sbb dx,0 ; negate
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finish:
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pop di
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pop si
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pop bp
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retf 8
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quick@ldiv:
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div bx ; unsigned divide
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; DX = remainder AX = quotient
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test di,2 ; want remainder?
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jz quick@quo
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xchg ax,dx
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quick@quo:
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xor dx,dx
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jmp short finish
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_TEXT ends
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end
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