Apple Firmware Volume Information (Type 128)

Source: the AppleSmBios.h header of OpenCore, Include/Apple/IndustryStandard.

Tells the features of the firmware, which macOS and its boot loader read, and the layout of its flash memory. The features are 64 bits wide: the structure holds their low 32 bits, and newer firmware their high 32 bits as the extended features. apple/imac11-3.bin carries the structure of 88 bytes, without the extended features.

Table 1. Apple Firmware Volume Information structure
Offset	Version	Name	Length	Value	Description
`0x00`	2.0+	Type	`BYTE`	`128`	Apple Firmware Volume Information indicator
`0x01`	2.0+	Length	`BYTE`	Varies	Length of the structure
`0x02`	2.0+	Handle	`WORD`	Varies
`0x04`	2.0+	Number of Regions	`BYTE`	Varies	Number of the regions of the flash memory
`0x05`	2.0+	Reserved	3 `BYTE`	`0`
`0x08`	2.0+	Firmware Features	`DWORD`	Bit field	Features of the firmware, bits 0 to 31, see Firmware Features
`0x0C`	2.0+	Firmware Features Mask	`DWORD`	Bit field	Features the firmware tells
`0x10`	2.0+	Region Type Map	8 `BYTE`	Enum	Types of the regions, see Region Types
`0x18`	2.0+	Flash Map	64 `BYTE`	Varies	Start and end addresses of the regions, a `DWORD` each
`0x58`	2.0+	Extended Firmware Features	`DWORD`	Bit field	Features of the firmware, bits 32 to 63, on newer firmware
`0x5C`	2.0+	Extended Firmware Features Mask	`DWORD`	Bit field	Extended features the firmware tells, on newer firmware

1. Apple Firmware Volume Information — Firmware Features

Table 2. Apple Firmware Volume Information: Firmware Features
Bit	Meaning
0	Supports booting legacy operating systems (CSM)
1	Supports booting from optical drives
2	Supports target disk mode
4	Supports network boot
5	Supports the recovery embedded into the firmware (SlingShot)
8	Supports wireless networks in the firmware
10	Bit 0 of the platform security policy
11	Bit 1 of the platform security policy
12	Firmware is locked
14	Supports USB 2.0 in the firmware
17	USB ports are not substituted with others by macOS
19	Supports APFS
20	Supports APFS, unused by the known Macs
22	Supports Apple tamper resistant boot X
24	Supports the platform security policy
25	Supports the extended features
28	Memory map is not adjusted by the boot loader on hibernation wake
29	Supports UEFI Windows boot
31	Memory map is not adjusted by the boot loader for the boot script
35	Supports the large BaseSystem, which macOS 12 requires

The other bits are of no known meaning.

2. Apple Firmware Volume Information — Region Types

Table 3. Apple Firmware Volume Information: Region Types
Value	Meaning
`0`	Entry is not used
`1`	Recovery firmware
`2`	Main firmware
`3`	Non-volatile variables (NVRAM)
`4`	Configuration
`5`	Diagnostics vault