TRANSPORTUIP - Microcode Subroutine Call Setup

Purpose

TRANSPORTUIP prepares for microcode subroutine execution by saving a return address. It does NOT perform the actual jump - a subsequent U_JCC or U_JMP instruction does that. Return from subroutine happnes via U_JMP_INDIR or using some flow bits (xxx need to clarify this).

Operation

result = TRANSPORTUIP(CONST, target_label, IA.11, U3.01)

• Saves the current microcode position as a return address
• Returns some value in result register (internal state, not the address itself)
• The target_label is used by the following jump instruction
• When the subroutine ends with EOM (with special "subroutine return" flag), execution returns to the saved position

Example

TMP8 = TRANSPORTUIP(CONST, part_macro_fxsave_alt, IA.11, U3.01)
// TMP8 contains internal state (not directly usable)
// Actual jump happens next (implicitly or explicitly)

Subroutine Return

• Subroutines end with EOM that has a special flag bit set
• This flag indicates "return to saved address" instead of "end of macro-operation"
• The return address comes from what TRANSPORTUIP saved internally
• Normal EOM (without flag) means end of entire instruction

Example from FXSAVE

// Main path:
TMP8 = TRANSPORTUIP(CONST, part_macro_fxsave_alt, IA.11, U3.01)
[... store FP registers ...]

// At part_macro_fxsave_alt:
[... alternate FP register stores ...]
EOM + ???FL2??? // With return flag - goes back to main path

Cpuid in some cache CRBUS handling subroutine also uses U_JMP.INDIR to return back from subroutine.

Notes

• The returned value is not used directly in code but has some meaning
• Internal mechanism likely uses a microcode call stack (or register chain?)
• Allows code reuse and conditional path selection in microcode

OLDER NOTES:

Typical micro-subroutine call

Example with TMP5 used as return address from subroutine:

...
TMP5 =         TRANSPORTUIP   (CONST         , loc_ret  , IA_11, U3_01)
...
U_JMP_NT       (CONST         , sub_xyz  , IA_11, U3_1B)
loc_ret:
...


# microcode subroutine
sub_xyz:
...
U_JMP_INDIR_N  (CONST_0       , TMP5          , U3_40)