Memory Management

Types and structures

enum MemoryOperation

Memory operation	Id
FREE	1
RESERVE	2
COMMIT	3
MAP	4
UNMAP	5
PROTECT	6
REGION APP	0x100
REGION SYSTEM	0x200
REGION BASE	0x300
LINEAR	0x10000

The LINEAR memory-operation indicates that the mapped physical address is always MappedVAddr+0x0C000000, thus this memory can be used for hardware devices' DMA(such as the GPU). Addr0+size for this must be within the 0x14000000-0x1C000000 range when Addr0 is non-zero(Addr1 must be zero), Addr0 isn't actually used by svcControlMemory for mapping memory: Addr0 is not used by the kernel after doing address-range checks. The kernel determines what physical-address to use by allocating memory from FCRAM(about the same way as other memory), which is then used to determine the virtual-address.

8.0.0-18 added a new memory mapping(0x30000000-0x38000000) for LINEAR memory, this replaces the original mapping for newer titles. The kernel uses the new mapping when the process memory-region is BASE, or when the process kernel-release-version field is >=0x022c(2.44 / system-version 8.0.0-18).

The input mem-region value for svcControlMemory is only used(when non-zero) when the PID is value 1, for the FIRM ARM11 "loader" module.

enum MemoryPermission

Memory permission	Id
NONE	0
R	1
W (Invalid, see below)	2
RW	3
X	4
RX	5
WX	6
RWX	7
DONTCARE	0x10000000

Specifying write-permission without read-permission is invalid, it's handled the same way as if the RW bits were not set(ARM11-MPCore doesn't support write-only memory permissions).

enum MemoryState

Memory state flags	Value
FREE	0
RESERVED	1
IO	2
STATIC	3
CODE	4
PRIVATE	5
SHARED	6
CONTINUOUS	7
ALIASED	8
ALIAS	9
ALIAS CODE	10
LOCKED	11

struct MemoryInfo

Type	Field
u32	Base process virtual address
u32	Size
u32	Permission
enum MemoryState	State

enum PageFlags

Page flags	Bit
LOCKED	0
CHANGED	1

struct PageInfo

Type	Field
PageFlags (u32)	Flags

Memory Mapping

ControlMemory and MapMemoryBlock can be used to map memory pages, these two SVCs only support mapping execute-never R/W pages. The input permissions parameter for these SVCs must therefore be <=3, where value zero is used when un-mapping memory. Furthermore it appears that only regular heap pages can be mirrored (it won't work for TLS, stack, .data, .text, for example).

Bitmask 0xF00 for ControlMemory parameter MemoryType is the memory-type, when this is zero the memory-type is loaded from the kernel flags stored in the exheader ARM11 kernel descriptors, for the process using the SVC.

ControlMemory parameter MemoryType with value 0x10003 is used for mapping the GSP heap. The low 8-bits are the type: 1 is for un-mapping memory, 3 for mapping memory. Type4 is used to mirror the RW memory at Addr1, to Addr0. Type4 will return an error if Addr1 is located in read-only memory. Addr1 is not used for type1 and type3.

The ARM11 kernel does not allow processes to create shared memory blocks via svcCreateMemoryBlock, when the process memorytype (from the kernel flags stored in the exheader kernel descriptor) is the application memorytype, and when addr=0. When the memorytype is not the application memorytype and addr=0, the kernel allocates new memory for the calling process and turns it into a shared memory block. When addr is non-zero, it must be located in memory which is already mapped. Furthermore, it appears that only regular heap pages (allocated using svcControlMemory op=COMMIT) are accepted as valid addrs. The addr(+size) must be >=0x00100000 and <0x14000000.

ControlProcessMemory maps memory in the specified process, this is the only SVC which allows mapping executable memory. Format of the permissions field for memory mapping SVCs: bit0=R, bit1=W, bit2=X. Type6 sets the Addr0 memory permissions to the input permissions, for already mapped memory. Type is the MemoryOperation enum, without the memory-type/memory-region. ControlProcessMemory only supports type4, type5, and type6. ControlProcessMemory does not support using the current KProcess handle alias.

Note that with the MAP MemoryOperation, the kernel will refuse to MAP memory for the specified addr1, when addr1 was already used with another MAP operation as addr1. The kernel also doesn't allow memory to be freed via the FREE MemoryOperation, when other virtual-memory is mapped to this same memory(when the MAP MemoryOperation was used with this memory with addr1). With the MAP MemoryOperation, the memory permissions for the original buffer are also set to no-access.

MapProcessMemory(Handle process, u32 destAddr, u32 limit) maps memory from the given process into the current process. This memory is mapped with permissions RW-, regardless of the permissions for the memory under the specified process. First min(limit, 0x3F00000) bytes are mapped starting from 0x00100000 in the source process to destAddr in the current process. Then min(limit - 0x7F00000, 0x6000000) bytes (if more than 0) are mapped from 0x08000000 in the source process to destAddr + 0x7F00000 in the current process. Another way to view this is that it is overlaying the two ranges [0x0010_0000; 0x0400_0000] and [0x0800_0000; 0x0E00_0000] from the source process onto [destAddr - 0x100000; destAddr + limit] in the current process, truncating whatever part of the mapping that doesn't fit. This system call is used by RO Services to map the program's code and heap into ro. Memory mapped by MapProcessMemory is unmapped by UnmapProcessMemory.

How The Kernel Allocates And Tracks Memory

FCRAM (128MiB for O3DS, or 256MiB for N3DS) is divided into three regions: APPLICATION, SYSTEM and BASE. A program is allowed to allocate memory in one of the three region. (For example games are always in the APPLICATION region). Inside one region, there are two kinds of memory that can be allocated: (regular) heap and linear heap.

(Regular) heap is allocated starting from the end of the memory region, and growing down. They are mostly for application private use, and sometimes for software-based memory sharing (with other process using KSharedMemory, for example). They can be mapped to anywhere inside 0x08000000~0x10000000 virtual memory range upon application request. When the application requests for a block of heap, the block is not guaranteed continuous in FCRAM and the location is not specified either.

Linear heap is allocated starting from the beginning of the memory region, and growing up. They are mostly for hardware-based memory sharing (with GPU, DMA etc.), but can also be used privately as well. They can be mapped to the linear heap virtual memory region (0x14000000+ or 0x30000000+, depending on game's kernel version). When the application requests for a linear heap block, the block is always continuous in FCRAM, and the difference between physical address and virtual address is always a constant. Therefore it gives virtual to physical address convertibility, which enables communication with other hardware.

Each region in the kernel is managed by a dedicated KPageHeap object, which tracks free memory blocks. Each free block is represented with a KPageHeapBlock structure stored in FCRAM that stores the size of the free region and links to adjacent blocks. When a chunk of memory is allocated from the heap, the block is removed from the linked list and the region is cleared, erasing the header. On initialization a free block is inserted that covers the entire region.

KPageHeapBlock

Size: 0xC-bytes(?) pre-v11.0, 0x18-bytes starting with v11.0.

Offset	Type	Description
0x0	u32	Size in pages
0x4	KPageHeapBlock*	Next
0x8	KPageHeapBlock*	Prev
0xC	KPageHeapBlock*	Pointer to the current memchunk. Added with v11.0?
0x10	u32	Nonce, doesn't seem to be read at all except during MAC calculation. Added with v11.0. Used with the new heap security feature. A kernel state field is copied to this field before calculating the MAC. Once done, that kernel state field is subtracted by the value of the calculated MAC stored below. Since this kernel state field is initially 0x0, this field for the FCRAM APPLICATION+0 KPageHeapBlock during kernel boot is set to 0x0.
0x14	u32	MAC calculated over rest of struct. Added with v11.0. Used with the new heap security feature.

KPageHeap

Size: 0x10-bytes pre-11.0.0-X, 0x20-bytes starting with 11.0.0-X.

Offset	Type	Description
0x0	KPageHeapBlock*	First
0x4	KPageHeapBlock*	Last
0x8	u32	Region start
0xC	u32	Region size
0x10	0x10-byte block.	This is the "key" used with the kernel heap MAC implemented with 11.0.0-X.

The KPageHeap objects are owned by the KMemoryManager. This object acts as a light wrapper that abstracts allocations and freeing of memory from the heaps and handles page reference tracking.

KMemoryManager

Size: 0x50-bytes pre-11.0.0-X, 0x80-bytes starting with 11.0.0-X.

X = KPageHeap_size*3.

Offset	Type	Description
0x0	KPageHeap	KPageHeap for app memory
KPageHeap_size*1	KPageHeap	KPageHeap for sys memory
KPageHeap_size*2	KPageHeap	KPageHeap for base memory
X + 0x0	KMemoryManager*	Ptr to start of FCRAM region descriptor
X + 0x4	u32	FCRAM start
X + 0x8	u32	FCRAM size in pages
X + 0xC	u32*	Pointer to FCRAM memory used for page reference tracking. Each u32 represents a page.
X + 0x10	u32	Count of physical FCRAM used by the kernel, in bytes. (used by svcGetSystemInfo)
X + 0x14	u32	?
X + 0x18	KLightMutex	Mutex used for thread synchronization during memory (de)allocation from the heaps.