It both directly executes from ROM and copies stuff into RAM.
On a modern x86 processor, the chipset memory controller is uninitialized at initial power-up, so there is no RAM available.
A modern BIOS is usually divided into two parts:
Boot Block (early POST)
Compressed Area (mid-to-late POST)
When the processor comes out of reset, it begins executing instructions at a fixed address in memory, called the "reset vector". The BIOS flash chip is mapped to this address in memory. The processor simply starts executing instructions from this address.
The "Boot Block" refers to the BIOS code starting at the reset vector (plus a few JMPs). This is executed directly from ROM (memory controller isn't up yet), so it is very slow.
The BIOS Boot Block generally does the following:
Initialize the memory controller. (If you get a "memory not detected" beep code from a motherboard, it happens here.)
Perform a checksum on the Compressed Area to make sure the rest of the BIOS is free of corruption.
Jump into a Recovery Mode if any corruption is detected.
If the checksum passes, decompress the rest of the BIOS into RAM somewhere (typically below the 1MB boundary).
Jump to the decompressed code in RAM and continue with boot.
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它既直接从 ROM 执行并且将内容复制到 RAM 中。
在现代 x86 处理器上，芯片组内存控制器在初始加电时未初始化，因此没有可用的 RAM。
现代 BIOS 通常分为两部分：
当处理器退出复位状态时，它开始在内存中的固定地址（称为“复位向量”）执行指令”。 BIOS闪存芯片被映射到内存中的这个地址。处理器只是从该地址开始执行指令。
“引导块”是指从复位向量（加上一些 JMP）开始的 BIOS 代码。这是直接从 ROM 执行的（内存控制器尚未启动），因此速度非常慢。
BIOS 引导块通常执行以下操作：
It both directly executes from ROM and copies stuff into RAM.
On a modern x86 processor, the chipset memory controller is uninitialized at initial power-up, so there is no RAM available.
A modern BIOS is usually divided into two parts:
When the processor comes out of reset, it begins executing instructions at a fixed address in memory, called the "reset vector". The BIOS flash chip is mapped to this address in memory. The processor simply starts executing instructions from this address.
The "Boot Block" refers to the BIOS code starting at the reset vector (plus a few JMPs). This is executed directly from ROM (memory controller isn't up yet), so it is very slow.
The BIOS Boot Block generally does the following:
如果您完全移除 RAM，电脑在开机时应该会开始不断发出蜂鸣声。
以下是对所发生情况的一个很好的描述：
http://www.pcguide.com/ref/mbsys/bios/boot.htm
Hans-Peter Messmer 的“The Indespensible PC Hardware Book”也描述了这个过程。
If you remove the RAM completely the PC should start to beep incessantly when you turn it on.
Here is a good description of what happens:
http://www.pcguide.com/ref/mbsys/bios/boot.htm
There's also Hans-Peter Messmer's 'The Indespensible PC Hardware Book' that describes the process too.