Why does system report less memory in UEFI mode than in legacy BIOS?
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I noticed that on a machine, if the firmware is configured in UEFI mode instead of Legacy BIOS, the operating system reports slightly less total memory (this is on Linux, but that shouldn't make a difference?). The difference is very small, about 3.5 MB, but I wonder, where does this memory go?
My understanding is that the Legacy BIOS compatibility mode is not actually a "real" BIOS, but rather a compatibility shim layer that the firmware loads to make it look to the OS like it's using a BIOS. All the UEFI code is still there. So there should be no difference, or if there is one, the difference should be the other way since the BIOS compatibility code uses a tiny bit of memory.
memory bios uefi
asked 21 hours ago
janneb
945612
add a comment |
up vote
0
down vote
favorite
I noticed that on a machine, if the firmware is configured in UEFI mode instead of Legacy BIOS, the operating system reports slightly less total memory (this is on Linux, but that shouldn't make a difference?). The difference is very small, about 3.5 MB, but I wonder, where does this memory go?
My understanding is that the Legacy BIOS compatibility mode is not actually a "real" BIOS, but rather a compatibility shim layer that the firmware loads to make it look to the OS like it's using a BIOS. All the UEFI code is still there. So there should be no difference, or if there is one, the difference should be the other way since the BIOS compatibility code uses a tiny bit of memory.
memory bios uefi
asked 21 hours ago
janneb
945612
1
Speculation : The UEFI might be loading 'extra' software. Some user application or some vendor specific thing. Or maybe it's just the graphical drivers to run to the UEFI look?
– Ricardo S.
20 hours ago
2
It could be that there are two things going on here. Aside from the BIOS shim taking up some memory there could be a UEFI shim that offers secure boot services (amongst others) to the operating system in order to provide the trusted execution platform facilities. It might be that this environment is 4MB, but the BIOS environment is only 512KB, which would account for the difference. Not certain, hence only a comment...
– Mokubai♦
19 hours ago
add a comment |
up vote
0
down vote
favorite
up vote
0
down vote
favorite
I noticed that on a machine, if the firmware is configured in UEFI mode instead of Legacy BIOS, the operating system reports slightly less total memory (this is on Linux, but that shouldn't make a difference?). The difference is very small, about 3.5 MB, but I wonder, where does this memory go?
My understanding is that the Legacy BIOS compatibility mode is not actually a "real" BIOS, but rather a compatibility shim layer that the firmware loads to make it look to the OS like it's using a BIOS. All the UEFI code is still there. So there should be no difference, or if there is one, the difference should be the other way since the BIOS compatibility code uses a tiny bit of memory.
memory bios uefi
asked 21 hours ago
janneb
945612
I noticed that on a machine, if the firmware is configured in UEFI mode instead of Legacy BIOS, the operating system reports slightly less total memory (this is on Linux, but that shouldn't make a difference?). The difference is very small, about 3.5 MB, but I wonder, where does this memory go?
My understanding is that the Legacy BIOS compatibility mode is not actually a "real" BIOS, but rather a compatibility shim layer that the firmware loads to make it look to the OS like it's using a BIOS. All the UEFI code is still there. So there should be no difference, or if there is one, the difference should be the other way since the BIOS compatibility code uses a tiny bit of memory.
memory bios uefi
memory bios uefi
asked 21 hours ago
janneb
945612
asked 21 hours ago
janneb
945612
asked 21 hours ago
janneb
945612
asked 21 hours ago
janneb
945612
asked 21 hours ago
janneb
945612
945612
1
Speculation : The UEFI might be loading 'extra' software. Some user application or some vendor specific thing. Or maybe it's just the graphical drivers to run to the UEFI look?
– Ricardo S.
20 hours ago
2
It could be that there are two things going on here. Aside from the BIOS shim taking up some memory there could be a UEFI shim that offers secure boot services (amongst others) to the operating system in order to provide the trusted execution platform facilities. It might be that this environment is 4MB, but the BIOS environment is only 512KB, which would account for the difference. Not certain, hence only a comment...
– Mokubai♦
19 hours ago
add a comment |
1
Speculation : The UEFI might be loading 'extra' software. Some user application or some vendor specific thing. Or maybe it's just the graphical drivers to run to the UEFI look?
– Ricardo S.
20 hours ago
2
It could be that there are two things going on here. Aside from the BIOS shim taking up some memory there could be a UEFI shim that offers secure boot services (amongst others) to the operating system in order to provide the trusted execution platform facilities. It might be that this environment is 4MB, but the BIOS environment is only 512KB, which would account for the difference. Not certain, hence only a comment...
– Mokubai♦
19 hours ago
1
1
Speculation : The UEFI might be loading 'extra' software. Some user application or some vendor specific thing. Or maybe it's just the graphical drivers to run to the UEFI look?
– Ricardo S.
20 hours ago
Speculation : The UEFI might be loading 'extra' software. Some user application or some vendor specific thing. Or maybe it's just the graphical drivers to run to the UEFI look?
– Ricardo S.
20 hours ago
2
2
It could be that there are two things going on here. Aside from the BIOS shim taking up some memory there could be a UEFI shim that offers secure boot services (amongst others) to the operating system in order to provide the trusted execution platform facilities. It might be that this environment is 4MB, but the BIOS environment is only 512KB, which would account for the difference. Not certain, hence only a comment...
– Mokubai♦
19 hours ago
It could be that there are two things going on here. Aside from the BIOS shim taking up some memory there could be a UEFI shim that offers secure boot services (amongst others) to the operating system in order to provide the trusted execution platform facilities. It might be that this environment is 4MB, but the BIOS environment is only 512KB, which would account for the difference. Not certain, hence only a comment...
– Mokubai♦
19 hours ago
add a comment |
1 Answer
1
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oldest
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2
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The big difference ultimately ends up being that UEFI almost invariably needs more reserved memory, because:
- It has more interfaces than the legacy BIOS. Examples include the much more complex RTC interface, the EFI variable interface, and the EFI capsule loader interface.
- It's usually a lot more code than the legacy BIOS, largely to support the extra ABI's.
- It runs in protected or long mode (instead of real mode like legacy BIOS), which can access all the system's memory, which in turn makes the developers a lot lazier about memory efficiency.
Most well written CSM interfaces (the legacy BIOS functionality in most UEFI implementations) will actually unload most or all of the UEFI code that isn't needed when you boot in legacy mode, so all that extra reserved memory gets freed up.
answered 8 hours ago
Austin Hemmelgarn
2,31918
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
The big difference ultimately ends up being that UEFI almost invariably needs more reserved memory, because:
- It has more interfaces than the legacy BIOS. Examples include the much more complex RTC interface, the EFI variable interface, and the EFI capsule loader interface.
- It's usually a lot more code than the legacy BIOS, largely to support the extra ABI's.
- It runs in protected or long mode (instead of real mode like legacy BIOS), which can access all the system's memory, which in turn makes the developers a lot lazier about memory efficiency.
Most well written CSM interfaces (the legacy BIOS functionality in most UEFI implementations) will actually unload most or all of the UEFI code that isn't needed when you boot in legacy mode, so all that extra reserved memory gets freed up.
answered 8 hours ago
Austin Hemmelgarn
2,31918
add a comment |
up vote
2
down vote
The big difference ultimately ends up being that UEFI almost invariably needs more reserved memory, because:
- It has more interfaces than the legacy BIOS. Examples include the much more complex RTC interface, the EFI variable interface, and the EFI capsule loader interface.
- It's usually a lot more code than the legacy BIOS, largely to support the extra ABI's.
- It runs in protected or long mode (instead of real mode like legacy BIOS), which can access all the system's memory, which in turn makes the developers a lot lazier about memory efficiency.
Most well written CSM interfaces (the legacy BIOS functionality in most UEFI implementations) will actually unload most or all of the UEFI code that isn't needed when you boot in legacy mode, so all that extra reserved memory gets freed up.
answered 8 hours ago
Austin Hemmelgarn
2,31918
add a comment |
up vote
2
down vote
up vote
2
down vote
The big difference ultimately ends up being that UEFI almost invariably needs more reserved memory, because:
- It has more interfaces than the legacy BIOS. Examples include the much more complex RTC interface, the EFI variable interface, and the EFI capsule loader interface.
- It's usually a lot more code than the legacy BIOS, largely to support the extra ABI's.
- It runs in protected or long mode (instead of real mode like legacy BIOS), which can access all the system's memory, which in turn makes the developers a lot lazier about memory efficiency.
Most well written CSM interfaces (the legacy BIOS functionality in most UEFI implementations) will actually unload most or all of the UEFI code that isn't needed when you boot in legacy mode, so all that extra reserved memory gets freed up.
answered 8 hours ago
Austin Hemmelgarn
2,31918
The big difference ultimately ends up being that UEFI almost invariably needs more reserved memory, because:
- It has more interfaces than the legacy BIOS. Examples include the much more complex RTC interface, the EFI variable interface, and the EFI capsule loader interface.
- It's usually a lot more code than the legacy BIOS, largely to support the extra ABI's.
- It runs in protected or long mode (instead of real mode like legacy BIOS), which can access all the system's memory, which in turn makes the developers a lot lazier about memory efficiency.
Most well written CSM interfaces (the legacy BIOS functionality in most UEFI implementations) will actually unload most or all of the UEFI code that isn't needed when you boot in legacy mode, so all that extra reserved memory gets freed up.
answered 8 hours ago
Austin Hemmelgarn
2,31918
answered 8 hours ago
Austin Hemmelgarn
2,31918
answered 8 hours ago
Austin Hemmelgarn
2,31918
answered 8 hours ago
Austin Hemmelgarn
2,31918
2,31918
add a comment |
add a comment |
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1
Speculation : The UEFI might be loading 'extra' software. Some user application or some vendor specific thing. Or maybe it's just the graphical drivers to run to the UEFI look?
– Ricardo S.
20 hours ago
2
It could be that there are two things going on here. Aside from the BIOS shim taking up some memory there could be a UEFI shim that offers secure boot services (amongst others) to the operating system in order to provide the trusted execution platform facilities. It might be that this environment is 4MB, but the BIOS environment is only 512KB, which would account for the difference. Not certain, hence only a comment...
– Mokubai♦
19 hours ago