The original BIOS real-mode INT 13h interface supports drives of sizes up to about 8 GB using what is commonly referred to as "physical CHS addressing". This limit originates from the hardware interface of the IBM PC/XT disk hardware. The BIOS used the cylinder-head-sector (CHS) address given in the INT 13h call, and transferred it directly to the hardware interface. A lesser limit, about 504 MB, was imposed by the combination of CHS addressing limits used by the BIOS and those used by ATA hard disks, which are dissimilar. When the CHS addressing limits of both the BIOS and ATA are combined (i.e. when they are applied simultaneously), the number of 512-byte sectors that can be addressed represent a total of about 504 MB.