The XGA driver supports a 64KB, 1MB, or 4MB aperture into VRAM. However, the XGA presentation display driver does not use these apertures. Instead, it transfers bit maps from system memory to VRAM using the XGA driver's bus-master capabilities. The XGA driver has a sophisticated memory-management unit, and is capable of dealing with virtual addresses. In OS/2, there is no guarantee that a given page of memory will be resident in memory at any given time. Therefore, to transfer a bit map to VRAM by way of a bus-master, you must first lock it down. Because this is time-consuming, the XGA driver locks down 64KB of system memory and performs any bus-master operations to or from VRAM using this 64KB buffer. This 64KB of memory is referred to as the Phunk (physical chunk).

The Phunk is created in eddb_CreatePhunk (located in the EDDBPHNK.C file). First, eddb_CreatePhunk allocates 64KB of memory. It then executes the FlatLock macro, from the EDDMACRO.H file. The FlatLock macro is an IOCtl to the XGA ring 0 driver. The IOCtl call takes the virtual address of the 64KB buffer, calls the Dev_Hlp VMLock, which locks the buffer into system memory, and then returns the physical address of the 64KB buffer to the caller. VMLock is limited to locking down 64KB of contiguous memory, which explains the size of the Phunk.

The PixBltThroughClipsViaPhunk() function is the primary user of the Phunk and is located in PIXBLT.C. PixBltThroughClipsViaPhunk is called by eddb_BitBlt() (in EDDNBBLT.C), which is the driver entry point for BitBlt. The XGA driver uses the Phunk whenever the source bit map is in system memory and the destination is the video memory, or whenever the source bit map is in video memory and the destination is in system memory. In other words, if the Blt involves a transfer to or from the video memory to system memory, it goes through the Phunk. PixBltThroughClipsViaPhunk() reduces the transfer into 64KB-size pieces and performs clipping on the destination by enumerating through the clipping rectangles. It then calls whichever low-level blt function has been set up by eddb_BitBlt(). This process is repeated for each 64KB of the bit map.