How Bios Shadow Works?

Typically, BIOS is stored on a motherboard within ROM chips—either EPROMs in older systems or flash chips in newer systems. Compared to RAM, these ROM chips are slower and have limited access speeds (150ns to 200ns for ROM versus 60ns to 70ns for RAM), and data is accessed in smaller parts (8 bits at a time in ROM versus 32 bits in RAM on systems like the 486 or 386DX). Here is how the BIOS Shadowing works –

This process is enabled through BIOS settings that allow shadowing during the initial boot sequence. Once the BIOS code is copied into RAM, the system begins to execute BIOS functions directly from RAM rather than ROM. This shift to a faster type of memory accelerates the execution of BIOS-related tasks, thereby speeding up the overall boot process and enhancing system performance. Additionally, by reducing the number of read cycles on the ROM, BIOS shadowing can potentially extend the lifespan of the ROM chip.

During boot-up, if shadowing is enabled (often via a CMOS setup option), the BIOS code is copied from the ROM to an equivalent memory range in the RAM (specifically in the 64KB range of F000-FFFF where the ROM BIOS is accessed). Enabling system BIOS shadowing is generally recommended to boost performance. However, with the development of faster ROM technologies and more advanced firmware like UEFI, the need for BIOS shadowing has decreased.

The term BIOS shadow refers to a technique used in computing to increase the speed of the system’s BIOS (Basic Input/Output System) operations. It enhances performance by copying ROM (random-access memory) contents to the RAM, so the CPU (central processing unit) may access the information quickly.

The primary advantage of this method is the increased speed of BIOS operations, which can significantly improve system startup times and the performance of certain low-level functions. While BIOS shadowing was an important optimization in the past, modern computers with more advanced hardware and firmware, such as UEFI (Unified Extensible Firmware Interface), no longer require such processes, as they are inherently more efficient.