AMD 29000 (was Re: TI99: WP and PC)

Tony Duell <ard(a)p850ug1.demon.co.uk> wrote: The AMD 29000 family of RISC processors have something similar, but arguably even better. It's also similar to register windows as used on processors like the SPARC, but it's more flexible. They also use 8-bit fields for register access. Only a few special registers are in the range of 0..63. 64..127 are global registers. But registers 128..255 are local registers, and the low seven bits of one of the special registers, the local register pointer, is added to the low seven bits of any register address in that range. The high 25 bits of the local register pointer are not used by the processor, but they play a key role in the software. The local registers are used as a cache of the top stack frames. Only complete stack frames can reside in the local registers, which means that the maximum size of a stack frame is 128 32-bit words. When a non-leaf function is called, the function prolog tests whether there are a sufficient number of local registers available for its frame. If not, one or more of the oldest stack frames are spilled to memory to make room. Then the local register pointer is updated so that the new stack frame starts at local register 128. Note that even stack frames in the local registers have a corresponding area in memory associated with them, into which they might get spilled by another function call. Thus the local register pointer always contains the full 32-bit address of the memory assigned to the current stack frame. The really clever thing that they did in the 29000 was that the function prolog was only about three instructions long. One of the instructions was an assert, which would cause a trap if a spill was necessary. But if no spill was necessary, the assert only takes a single CPU cycle. Similarly there is a epilog on function exit containing an assert that will trap to fill an old stack frame back from memory into the local registers if necessary. The down side of all this is that if you need to do a context switch, you have to save a huge number of registers. There was a clever hack to avoid needing to save too many registers in interrupt handlers. Note that an interrupt can occur during a function prolog or epilog, though not in the spill or fill traps. But because the function prolog and epilog are manipulating various pointers used by the spill/fill mechanism, when the interrupt occurs those registers may be in an inconsistent state. Daniel Mann of AMD wrote an application note in which he described a method for the interrupt handler to detect whether a prolog or epilog had been interrupted, and deal with it accordingly. But unfortunately I was not able to get it to work. It turns out that if you don't need the interrupt handler to be able to cause a context switch (i.e., you're not writing a preemptive OS kernel), you can ignore much of this complexity. The interrupt handler's assembly language glue can simply save the local register pointer, and load it with a new value that points to an interrupt stack, and makes it look like the local registers contain a maximum-sized stack frame, which I'll call the pseudo-frame. Note that the pseudo-frame may actually contain any number of user frames, and that they are not necessarily aligned in any particular order since the local register pointer has been changed. Now when the interrupt happens, the interrupt handler's function prolog will have to cause a spill in order to make room for its own stack frame. But because of the way the stack has been swapped, the needed registers will spill not to the task's stack, but the interrupt stack. Note that a spill does not necessarily write an entire stack frame to memory, but only enough registers to make room for the new frame. However, a fill has to make sure that at least one complete frame is available. When the interrupt handler returns, its epilog will fill the pseudo-frame back into the local registers, then the assembly language glue will reload the orginal values of the local register pointer. Another amusing feature of the 29000 is that some of AMD's software used global register 4. I was stunned when I noticed this, because the documentation denies the existence of global register 4. My own attempts to use global register 4 failed. It turns out that like most RISC processors (and even some CISCs), there is a feed-forward data path so that if the result of one instruction is used by the next instruction, that data is fed back around directly from the ALU output back to its input. Otherwise a pipeline stall would be necessary so that the second instruction wouldn't read stale data from the register file. Because of this feed-forward mechanism, it is possible to use any unimplmented register as a single-cycle temporary register. But this is only safe when interrupts are disabled, because if an interrupt were to happen after the first instruction of the pair, the data for the unimplemented register can't be saved. Eric