GSP Shared Memory

This page describes the structure of the GSP shared memory. GX commands and framebuffer info is stored here, and other unknown data. After writing the command data to GSP shared memory, TriggerCmdReqQueue must be used to trigger GSP processing for the command.

Framebuffer info

The framebuffer info structure for the main LCD is located at sharedmemvadr + 0x200 + threadindex*0x80. The framebuffer info structure for the sub LCD is located at sharedmemvadr + 0x240 + threadindex*0x80.

Framebuffer info header

Index Byte	Description
0	Framebuffer info entry index
1	Flag
3-2	Padding

When a process sets this framebuffer info, it sets index to (index+1) & 1. Then it writes the framebuffer info entry, and sets flag to value 1. The GSP module seems to load this framebuffer info entry data into GSP state once the GPU finishes processing GX commands 3 or 4. Once the GSP module finishes loading this framebuffer info, it sets flag to value 0, then it will not load the framebuffer info again until flag is value 1. After loading this entry data into GSP state, the GSP module then writes this framebuffer state to the LCD registers.

The two 0x1C-byte framebuffer info entries are located at framebufferinfo+4.

Command Buffer Header

Index Byte	Description
0	Command Index, must be <=15
1	Must not be value 0
2	Must not be value 1. When the error-code u32 is set, this u8 is set to value 0x80.
3	Bit0 must not be set
4	u32 Error code for the last GX command which failed

The command buffer is located at sharedmem + 0x800 + threadindex*0x200.

Command Header

Index Byte	Description
0	Command ID
2-1	?
3	When non-zero GSP module may check flags for the specified cmdID, command handling is aborted when the flags are set.

The command is located at cmdbuf + 0x20 + cmdindex*0x20, the size of each command is 0x20-bytes. The command parameters are located at command+4. Addresses specified in parameters are application vaddrs, these are usually located in either the process GSP heap or VRAM. For applications these addresses are normally located in the GSP heap, while for other processes these addresses are located in VRAM. Addresses/sizes specified in parameters except for cmd0 and cmd5 must be 8-byte aligned.

Commands

GX Command 0

Index Word	Description
0	u8 CommandID is 0x00
1	Source address
2	Destination address
3	Size
7-4	Unused

This command is normally used to DMA data from the application GSP heap to VRAM.

GX Command 1

Index Word	Description
0	u8 CommandID is 0x01
1	Buffer address
2	Buffer size
3	Flag, bit0 is written to GSP module state
6-4	Unused
7	When non-zero, call svcFlushProcessDataCache() with the specified buffer

This command converts the specified address to a physical address, then writes the physical address and size to the GPU registers at 0x1EF018E0. This buffer contains GPU commands.

GX Command 2

Index Word	Description
0	u8 CommandID is 0x02
1	Buf0 address
2	Buf0 size
3	Associated buf0 address
4	Buf1 address
5	Buf1 size
6	Associated buf1 address
7	The low u16 is used with buf0, while the high u16 is used with buf1

This commands converts the specified addresses to physical addresses, then writes these addresses and the specified parameters to the GPU registers at 0x1EF00010 and 0x1EF00020. The associated buffer address must not be <= to the main buffer address, thus the associated buffer address must not be zero as well. When the bufX address is zero, processing for the bufX parameters is skipped.

GX Command 3

Index Word	Description
0	u8 CommandID is 0x03
1	VRAM framebuffer address
2	Output framebuffer address
3	VRAM framebuffer dimensions
4	Output framebuffer dimensions
5	Unknown, for applications this is 0x1001000 for the main screen, and 0x1000 for the sub screen.
7-6	Unused

This command converts the specified addresses to physical addresses, then writes these physical addresses and parameters to the GPU registers at 0x1EF00C00. This command writes the rendered framebuffer data from the VRAM framebuffer address to the specified output framebuffer.

GX Command 4

Index Word	Description
0	u8 CommandID is 0x04
1	Buf0 address
2	Buf1 address
3	?
4	?
5	?
6	?
7	Unused

This command is similar to cmd3, this command also writes to the GPU registers at 0x1EF00C00.

GX Command 5

Index Word	Description
0	u8 CommandID is 0x05
1	Buf0 address
2	Buf0 size
3	Buf1 address
4	Buf1 size
5	Buf2 address
6	Buf2 size
7	Unused

The application buffer addresses specified in the parameters are used with svcFlushProcessDataCache. The input buf0 size must not be zero. When buf1 size is zero, svcFlushProcessDataCache() for buf1 and buf2 are skipped. When buf2 size is zero, svcFlushProcessDataCache() for buf2 is skipped.