The Mach layer at the bottom of the kernel is visible in userspace, but not directly used by most developers outside Apple.
内核底层的Mach层对于用户空间层（userspace）来说是可见的，但是却不能被大部分非苹果公司的开发者直接运用。
When the light is on, or white, the image is visible. When the light is off, or black, the layer can't be seen.
当在灯光下或白天，图像是可见的。当在灯熄或黑夜下，图像时不能被看见的。
This is the layer that is most visible to users and application providers.
用户和应用程序供应商看到最多的就是这一层。
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