32-bit line drawing (broken)

lines2.c

asm("mov ax, [bp - 4] \n"       // ax = (dy)
    "cwde \n");                 // sign extend ax to fill eax
    
asm("mov bx, [bp - 6] \n"       // bx = (newX)
    "xchg eax, ebx \n"          // ax = (newX) <--> bx = (dy)
    "cwde \n");                 // sign extend ax to fill eax
    
asm("imul ebx \n");             // eax = (signed)(dy * newX)
    
asm("mov edx, eax \n"           // edx = eax
    "mov ax, [bp - 2] \n"       // ax = (dx)
    "cwde \n");                 // sign extend ax to fill eax
    
asm("xchg edx, eax \n"          // eax = edx (eax = (dy * newX)) <-> edx = eax (dx)
    "xor edx, edx \n");         // edx is considered the most significant dword in the dividend
                                // ...clear it before dividing; only lower half is needed
asm("idiv edx \n");             // eax = (signed)((dy * newX) / dx)
    
asm("mov [bp - 8], ax");        // (newY) = ax
                                // only lower half of eax is needed as the quotient should not be
                                // ...any larger than a signed word