Super Harvard Architecture Single-Chip Computer
|This article relies too much on references to primary sources. (September 2010)|
The Super Harvard Architecture Single-Chip Computer (SHARC) is a high performance floating-point and fixed-point DSP from Analog Devices. SHARC is used in a variety of signal processing applications ranging from single-CPU guided artillery shells to 1000-CPU over-the-horizon radar processing computers. The original design dates to about January 1994.
SHARC processors are or were used because they have offered good floating-point performance per watt.
SHARC processors are typically intended to have a good number of serial links to other SHARC processors nearby, to be used as a low-cost alternative to SMP.
The SHARC is a Harvard architecture word-addressed VLIW processor; it knows nothing of 8-bit or 16-bit values since each address is used to point to a whole 32-bit word, not just a octet. It is thus neither little-endian nor big-endian, though a compiler may use either convention if it implements 64-bit data and/or some way to pack multiple 8-bit or 16-bit values into a single 32-bit word. Analog Devices chose to avoid the issue by using a 32-bit char in their C compiler.
The word size is 48-bit for instructions, 32-bit for integers and normal floating-point, and 40-bit for extended floating-point. Code and data are normally fetched from on-chip memory, which the user must split into regions of different word sizes as desired. Small data types may be stored in wider memory, simply wasting the extra space. A system that does not use 40-bit extended floating-point might divide the on-chip memory into two sections, a 48-bit one for code and a 32-bit one for everything else. Most memory-related CPU instructions can not access all the bits of 48-bit memory, but a special 48-bit register is provided for this purpose. The special 48-bit register may be accessed as a pair of smaller registers, allowing movement to and from the normal registers.
Off-chip memory can be used with the SHARC. This memory can only be configured for one single size. If the off-chip memory is configured as 32-bit words to avoid waste, then only the on-chip memory may be used for code execution and extended floating-point. Operating systems may use overlays to work around this problem, transferring 48-bit data to on-chip memory as needed for execution. A DMA engine is provided for this. True paging is impossible without an external MMU.
The SHARC has a 32-bit word-addressed address space. Depending on word size this is 16 GB, 20 GB, or 24 GB.
SHARC instructions may contain a 32-bit immediate operand. Instructions without this operand are generally able to perform two or more operations simultaneously. Many instructions are conditional, and may be preceded with "if condition " in the assembly language. There are a number of condition choices, similar to the choices provided by the x86 flags register.
There are two delay slots. After a jump, two instructions following the jump will normally be executed.
The SHARC processor has built-in support for loop control. Up to 6 levels may be used, avoiding the need for normal branching instructions and the normal bookkeeping related to loop exit.
The SHARC has two full sets of general-purpose registers. Code can instantly switch between them, allowing for fast context switches between an application and an OS or between two threads.