The Frame Buffer

Physical Address Space

The physical frame buffer is addressed as 2048×1024 bytes. The last 768 bytes of each line of the address space (those to the right of the screen) are not displayed and no memory exists in those areas.

Figure 4-1 Physical Address Space

To Access the Frame Buffer Directly

When using the R_GET_FRAME_BUFFER gescape for direct user access to the frame buffer, correct access can only be assured by using the R_LOCK_DEVICE and R_UNLOCK_DEVICE gescapes.

	CAUTION: Do not read from or write to the offscreen addresses. Such operations will cause errors.

Frame Buffer Address Mapping

The frame buffer is organized as a single one-dimensional array of pixel values. The first byte (byte 0) of the frame buffer represents the upper left corner pixel of the screen. Byte 1 is immediately to its right. Byte 1279 is the last (right-most) displayable pixel on the top line. The next 768 bytes are not displayable. Byte 2048 is the first (left-most) pixel on the second line from the top. The last (lower right corner) pixel on the screen is byte number 2,096,383 (1023×2048+1279).

Figure 4-2 Frame Buffer Mapping in Memory

The HP Visualize-FX² frame buffer has three banks of 8 planes each. The HP Visualize-FX⁴ and HP Visualize-FX⁶ frame buffers have six banks of 8 planes (two for each color). Only one bank can be accessed at a time. Use the bank_switch call to select a bank to read or write data directly from the frame buffer. For block_read and block_write operations to the image planes, the data is in all eight bits of each byte.

The default for reading the Z-buffer is always 24 bits per pixel in a 32-bit word. The Z-buffers for the HP Visualize-FX devices are 23 bits deep, and their Z-buffer data is left justified in the lower 24 bits of the 32-bit word (that is, the 23-bit Z-buffer data is shifted left one bit from the least-significant bit), as shown in the following figure.

Figure 4-3 Hardware Z-Buffer Data Alignment

The raw parameter to block_read and block_write must be set to true in order to read from or write to the Z-buffer. Using wbank=3 in the bank_switch command on the HP Visualize-FX² selects the Z-buffer for reads or writes. Using wbank=6 in the bank_switch command on the HP Visualize-FX⁴ and HP Visualize-FX⁶ selects the Z-buffer for reads or writes.

The 24 bits of the Z-buffer reside in the lower 24 bits of the word that is used by block_read and block_write.

Unlike the frame buffer, the Z-buffer data is contiguous. The HP Visualize-FX device's Z-buffers are always 1280×1024 where word 1280 is the leftmost word of the second scanline. For the HP Visualize-FX family of devices, the Z-buffer is the size of the window. For example, if the window is 400×400, word 400 is the leftmost Z-buffer value for the second scan line.

Frame Buffer Configurations

The following table shows which color map modes are supported for different frame buffer configurations.

Since Starbase supports double-buffering per window, it is better to request double-buffering with a depth of 24 when in CMAP_FULL mode on HP Visualize-FX⁴ and HP Visualize-FX⁶ devices, or with a depth of 12 when in CMAP_FULL mode on a HP Visualize-FX². Double-buffering with less than 8 planes (4/4, 3/3, 2/2, 1/1) is supported in depth 8 windows for compatibility with previous devices, however, it is not recommended. Note that the 4/4 double-buffering mentioned here does not use a depth 4 visual. The write_enable and display_enable masks are used to accomplish double-buffering with less than 8 planes. Video tearing may occur, however, as this kind of double-buffering is not synchronized to the video refresh.