Crusoe Exposed: Transmeta TM5xxx Architecture 2

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Branch Unit

Branches (both conditional and unconditional) within CMS use a 23 bit absolute target address aligned to a 64-bit boundary (i.e., abstarget is shifted left 3 bits). Interestingly, with 23 bits, 1 << (23+3) only allows up to 64 MB to be dedicated to CMS; this is probably one reason why larger translation caches have not been used.

CrusoeReport-Part2-fig5.gif - 18436 Bytes
Figure 5 – Crusoe Branch Instruction Format

It appears that the CMS address space is the only region from which code can be executed; the processor is physically incapable of executing code directly from user space. This makes sense considering that all x86 code must be translated (and thus copied to CMS space) before native execution.

Conditional branches use the exact same condition code set (cc bits) as the x86 encoding in jump instructions (see the Intel manuals). Unconditional branches can optionally write the return address to the %link register (%r58) if the L bit (bit 0 of the cc field) is set.

Indirect branches occur through a general purpose register. It appears that special instructions are provided to prepare for an indirect branch when the target address is known in advance; this avoids the three-cycle branch penalty. In addition, special instructions may provide a branch with link functionality.

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