[PATCH 4/5] OvmfPkg/README: document 4MB flash layout

[Laszlo Ersek <lersek@redhat.com>]

Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=527
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
---
 OvmfPkg/README | 39 ++++++++++++++++----
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/OvmfPkg/README b/OvmfPkg/README
index 304e69fbe545..33ff9432bb3e 100644
--- a/OvmfPkg/README
+++ b/OvmfPkg/README
@@ -245,15 +245,20 @@ longer.)
 
 === OVMF Flash Layout ===
 
-Like all current IA32/X64 system designs, OVMF's firmware
-device (rom/flash) appears in QEMU's physical address space
-just below 4GB (0x100000000).
+Like all current IA32/X64 system designs, OVMF's firmware device (rom/flash)
+appears in QEMU's physical address space just below 4GB (0x100000000).
 
-The layout of the firmware device in memory looks like:
+OVMF supports building a 1MB, 2MB or 4MB flash image (see the DSC files for the
+FD_SIZE_1MB, FD_SIZE_2MB, FD_SIZE_4MB build defines). The base address for the
+1MB image in QEMU physical memory is 0xfff00000. The base address for the 2MB
+image is 0xffe00000. The base address for the 4MB image is 0xffc00000.
+
+Using the 1MB or 2MB image, the layout of the firmware device in memory looks
+like:
 
 +--------------------------------------- 4GB (0x100000000)
 | VTF0 (16-bit reset code) and OVMF SEC
-| (SECFV)
+| (SECFV, 208KB/0x34000)
 +--------------------------------------- varies based on flash size
 |
 | Compressed main firmware image
@@ -271,9 +276,27 @@ The layout of the firmware device in memory looks like:
 | area (56KB/0xe000)
 +--------------------------------------- base address
 
-OVMF supports building a 1MB or a 2MB flash image. The base address for
-a 1MB image in QEMU physical memory is 0xfff00000. The base address for
-a 2MB image is 0xffe00000.
+Using the 4MB image, the layout of the firmware device in memory looks like:
+
++--------------------------------------- base + 0x400000 (4GB/0x100000000)
+| VTF0 (16-bit reset code) and OVMF SEC
+| (SECFV, 208KB/0x34000)
++--------------------------------------- base + 0x3cc000
+|
+| Compressed main firmware image
+| (FVMAIN_COMPACT, 3360KB/0x348000)
+|
++--------------------------------------- base + 0x84000
+| Fault-tolerant write (FTW)
+| Spare blocks (264KB/0x42000)
++--------------------------------------- base + 0x42000
+| FTW Work block (4KB/0x1000)
++--------------------------------------- base + 0x41000
+| Event log area (4KB/0x1000)
++--------------------------------------- base + 0x40000
+| Non-volatile variable storage
+| area (256KB/0x40000)
++--------------------------------------- base address (0xffc00000)
 
 The code in SECFV locates FVMAIN_COMPACT, and decompresses the
 main firmware (MAINFV) into RAM memory at address 0x800000. The
-- 
2.9.3