GX State Issues

State Issuing vs. the Display List

In this document we describe the following items:

• The GX functions that affect the same GP registers
• The GX functions that leave a state pending
• The GX functions that flush all pending states
• Types of registers affected by each GX function

This information can be used for various purposes, such as constructing display lists and optimizing function call patterns.

The following register types are shown in the tables below.

Note:  In the Revolution SDK, GX Library issues commands in a different order than in the GAMECUBE SDK; this serves to improve the internal processing efficiency of the graphics processor. Specifically, most states due to the XF command will be delayed (apart from a few exceptions, such as loading a matrix).

Well-Behaved GX Functions

We begin by describing the well-behaved GX functions. These functions each affect unique registers, and they insert their command tokens immediately (leaving no pending state). They are easy to put into display lists. 

Functions with Register Conflicts

These functions conflict with others that use the same register(s). They can be put into display lists carefully as long as you understand that doing so will affect all settings sharing the same registers. 

Note: Some of the register conflicts are also listed in the next section.

It is possible to work around any RS register conflict (it is not possible for CP or XF register conflicts). There is a mask register that can be used to write only certain bits within a RS register. Given that it takes time to set the mask register, none of the current GX functions use this feature. The one exception is the GXSetCoPlanar function (which would otherwise conflict with the genmode register). Future revisions of GX may include functions that utilize the mask register. If you think you can greatly benefit from using the mask register, please contact Developer Support with your reasons.

The "texture flush issue" means that a certain texture register must be written in order to force a texture system flush.
That register (imask) also happens to control which textures are direct and which are indirect. Refer to the next section (Note: information on imask is not included here).

Functions That Set Pending State

These functions can usually be placed in a display list, but there are various issues involved since these functions are not necessarily straightforward in their operation. 'Lazy' means that the register value is not sent right away, but only at the last moment. The following functions will flush any lazy (pending) state before they execute their intended task.

• GXBegin
• GXBeginDisplayList
• GXEndDisplayList
• GXCallDisplayList
• GXFlush

The scaling of texture coordinates is something that also occurs in a lazy fashion. The texture scale registers (suTs0 and suTs1) are per texture coordinate, not per texture. They are set based upon which texture coordinates are associated with which textures. Functions that can possibly affect this mapping thus trigger a flag to set these registers at the last moment, and this is indicated by 'scale issue' in the list below. This issue can be avoided by setting all of the scale values manually. The GX library allows this by calling the GXSetTexCoordScaleManually function.

The TMEM region issue means that these functions call the appropriate TMEM region allocator in order to know what region of TMEM to use. Putting these functions into display lists fixes the region of TMEM that is used. You must therefore deal with TMEM allocation carefully.

Be aware that the GXCopyDisp and GXCopyTex functions may possibly change various other registers (including pectrl, zmode and cmode0) depending upon the operation and what the registers (shadows) were before the operation. The old register shadow values are restored after the copy operation.

Other

The GXDraw* functions all get and restore the current VCD/VAT (based upon the GX shadow copies).

Note: The following GX functions should not be placed inside the display list. Operation is not guaranteed if these functions are included.

• GXInit
  
• GXAbortFrame
  
• GXDrawDone
  
• GXSetDrawDone
  
• GXWaitDrawDone
  
• GXBeginDisplayList
  
• GXEndDisplayList
  
• GXCallDisplayList
  
• GXFastCallDisplayList
  
• GXSetCPUFifo
  
• GXRedirectWriteGatherPipe
  
• GXRestoreWriteGatherPipe
  
• GXResetWriteGatherPipe
  

Revision History

2008/06/04 Added a list of functions that should not be placed in the display list.
2008/05/13 Added a precaution about the functions that may be placed in the display list.
2006/03/01 Initial version.

CONFIDENTIAL