Atari Jaguar II - Internal Blocks - Details Relative To Jaguar

Details Relative To Jaguar

This lists full details of the improvements, relative to the original Jaguar:

• There is an additional Jaguar RISC (J-RISC) processor, known as the RCPU, with a simple program cache. It is intended to perform the functionality of the CPU, acting as a geometry engine, and it is well suited to executing compiled code.
• The blitter can now draw polygons as a single operation. These may be just filled, or any combination of Gouraud shaded, Z-buffered, and texture mapped.
• The blitter can now draw texture maps at full bus speed  a maximum of one phrase per two clock cycles, from internal texture memory, and can also operate from external texture RAM more efficiently that before.
• The blitter can anti-alias textures as it renders them, using bi-linear filtering.
• The texture mapping and Gouraud shade modes can be combined to give shaded texture-mapped polygons, with Z-buffering as well if required. These can also be drawn at full bus speed. The shading is a multiplicative mix of the texture data and another colour, allowing lightening, darkening, distance-haze and other effects.
• The intensity calculations are now carried out with an extended range, using an eleven bit signed integer to represent intensity, this value being clipped (saturated) only when the pixels are drawn.
• A subset of the blitter registers are double-buffered, so that a polygon drawing engine can program the parameters for a polygon blit while the previous blit is still under way.
• There is no need to initialise all four I and Z values (or texture pointers) for a phrase mode blit, the blitter can automatically initialise them appropriately.
• The blitter address generators now both have clip window and mask functions. Formerly A1 had a clip window and A2 had a mask.
• The GPU has an overflow flag which reflects signed arithmetic overflow from add or subtract operations, and also gives the state of the bit modified by bit clear and set operations before the clear or set.
• The jump condition codes have been extended to cope with the new overflow flag, and now include all the conditions available on general purpose micro-processors, e.g. the 68000.
• The NOP instruction has been extended, so that if its operands are not zero then it becomes an unconditional jump relative with a ten bit signed jump offset, giving an increased range.
• Byte and word transfers to GPU RAM are now possible.
• The J-RISC processors all contain a simple DMA transfer engine, which allows full bus rate phrase mode transfers between internal and external memory. This speeds up program loads and data set transfers.
• The PACK and UNPACK instruction can now operate on RGB16 pixels as well as CRY.
• The object processor can now clip at a right hand side value of less than 720 by setting the limit register.
• The object processor can force the select bit for mixed CRY/RGB screens on a per-object basis.
• The object processor supports line-doubling so that a TV picture can be displayed on a progressive VGA monitor.
• The object processor can multiplicatively mix the pixel color with a “fade to” color according to a mix control value. A new object type defines the mixer control value and the mixed color.
• RMW objects can now have double the “strength”.
• Scaled objects may now be controlled to a higher precision, and the horizontal remainder may now be defined.
• Some additional extended jump condition codes allow debug functions, such as interrupt, stop and pause.

In addition, some bugs that created problems for Jaguar One programmers have been fixed:

• Score-board protection for writes is available, so that writes do not occur out of order. This is enabled by the GPU enhanced mode bit.
• GPU code can be executed out of external RAM.
• The blitter address flags for Y add control are now properly differentiated, there is an enable bit in the Collision control and Mode register that has to be set to fix this bug.
• The data register of an indexed store instruction now has full score-board protection.
• Problems related to MOVEI instructions at the beginning of a program, particularly when single stepping, have been resolved.
• Unscaled objects are now fetched at full bus speed.
• The pixel pre-scaler is now reset on the last line of the display, so the display need not be over-scanned to conceal it.
• Two divides may follow each other when one uses the quotient of another.
• The DSP external DMA interface has been completely overhauled, and will now support low and high priority transfers; as well as arbitrary load/store combinations and alignments.
• A variety of problems related to blitter window clipping have been resolved.