Question

On 16 bit dos machine there is options like FP_SEG and FP_OFF for converting a pointer to linear address but since these method no more exist on 32 bit compiler what are other function that can do same on 32 bit machine??

Answer 1

They’re luckily not needed as 32-bit mode is unsegmented and hence the addresses are always linear (a simplification, but let’s keep it simple).

EDIT: The first version was confusing let’s try again.

In 16-bit segmented mode (I’m exclusively referring to legacy DOS programs here, it’ll probably be similar for other 16-bit x86 OSes) addresses are given in a 32-bit format consisting of a16-bit segment and a 16-bit offset. These are combined to form a 20-bit linear address (This is where the infamous 640K barrier comes from, 2**20 = 1MB and 384K are reserved for the system and bios leaving ~640K for user programs) by multiply the segment by 16 = 0x10 (equivalent to shifting left by 4) and adding the offset. I.e.: linear = segment*0x10 + offset.

This means that 2**12 segment:address type pointers will refer to the same linear address, so in general there is no way to obtain the 32-bit value used to form the linear address.

In old DOS programs that used far – segmented – pointers (as opposed to near pointers, which only contained an offset and implicitly used ds segment register) they were usually treated as 32-bit unsigned integer values where the 16 most significant bits were the segment and the 16 least significant bits the offset. This gives the following macro definitions for FP_SEG and FP_OFF (using the types from stdint.h):

#define FP_SEG(x) (uint16_t)((uint32_t)(x) >> 16) /* grab 16 most significant bits */ 
#define FP_OFF(x) (uint16_t)((uint32_t)(x)) /* grab 16 least significant bits */

To convert a 20-bit linear address to a segmented address you have many options (2**12). One way could be:

#define LIN_SEG(x) (uint16_t)(((uint32_t)(x)&0xf0000)>>4)
#define LIN_OFF(x) (uint16_t)((uint32_t)(x))

Finally a quick example of how it all works together:

Segmented address: a = 0xA000:0x0123
As 32-bit far pointer b = 0xA0000123
20-bit linear address: c = 0xA0123

FP_SEG(b) == 0xA000
FP_OFF(b) == 0x0123
LIN_SEG(c) = 0xA000
LIN_OFF(c) = 0x0123