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[libreriscv.git] / simple_v_extension / sv_prefix_proposal.rst

diff --git a/simple_v_extension/sv_prefix_proposal.rst b/simple_v_extension/sv_prefix_proposal.rst
index 0dff42be519b490144a89d1fe3cab8fb810b5297..8260fee9e3630bbe5758c6fad7066a0ee960c24c 100644 (file)
--- a/simple_v_extension/sv_prefix_proposal.rst
+++ b/simple_v_extension/sv_prefix_proposal.rst
@@ -1,3 +1,5 @@
+[[!tag standards]]
+
 SimpleV Prefix (SVprefix) Proposal v0.3
 =======================================
 
@@ -12,6 +14,7 @@ into 32, 48 and 64 bit RV formats, to provide Vectorisation context
 on a per-instruction basis.
 
 .. _Specification: http://libre-riscv.org/simple_v_extension/specification/
+.. _Appendix: http://libre-riscv.org/simple_v_extension/appendix/
 
 .. contents::
 
@@ -215,7 +218,10 @@ prefix as well.  VLtyp encodes how (whether) to set SVPSTATE.VL and SVPSTATE.MAX
 VLtyp field encoding
 ====================
 
-NOTE: VL and MVL below are local to SVPregix and, if non-default, are tracked through SVPSTATE, not the main Specification_ STATE. If default (all zeros) then STATE VL and MVL apply to this instruction.
+NOTE: VL and MVL below are local to SVPrefix and, if non-default,
+will update the src and dest element offsets in SVPSTATE, not the main
+Specification_ STATE. If default (all zeros) then STATE VL and MVL apply
+to this instruction, and STATE.srcoffs (etc) will be used.
 
 +-----------+-------------+--------------+----------+----------------------+
 | VLtyp[11] | VLtyp[10:6] | VLtyp[5:1]   | VLtyp[0] | comment              |
@@ -229,7 +235,9 @@ NOTE: VL and MVL below are local to SVPregix and, if non-default, are tracked th
 | 1         |  VLdest     |  MVL-immed   | 1        | MVL immed mode       |
 +-----------+-------------+--------------+----------+----------------------+
 
-Note: when VLtyp is all zeros, the main Specification_ VL and MVL apply to this instruction. If called outside of a VBLOCK or if sv.setvl has not set VL, the operation is "scalar".
+Note: when VLtyp is all zeros, the main Specification_ VL and MVL apply
+to this instruction. If called outside of a VBLOCK or if sv.setvl has
+not set VL, the operation is "scalar".
 
 Just as in the VBLOCK format, when bit 11 of VLtyp is zero:
 
@@ -258,8 +266,38 @@ Note that VLtyp's VL and MVL are not the same as the main Specification_
 VL or MVL, and that loops will alter srcoffs and destoffs in SVPSTATE in VLtype nondefault mode, but the srcoffs and destoffs in STATE, if VLtype=0.
 
 Furthermore, the execution order and exception handling must be exactly
-the same as in the main spec
-(Program Order must be preserved)
+the same as in the main spec (Program Order must be preserved)
+
+Pseudocode for SVPSTATE.VL:
+
+.. parsed-literal::
+
+    # pseudocode
+
+    regs = [0u64; 128];
+    vl = 0;
+
+    // instruction fields:
+    rd = get_rd_field();
+    vlmax = get_immed_field();
+
+    // handle illegal instruction decoding
+    if vlmax > XLEN {
+        trap()
+    }
+
+    // calculate VL
+    if rs1 == 0 { // rs1 is x0
+        vl = vlmax
+    } else {
+        vl = min(regs[rs1], vlmax)
+    }
+
+    // write rd
+    if rd != 0 {
+        // rd is not x0
+        regs[rd] = vl
+    }
 
 vs#/vd Fields' Encoding
 =======================
@@ -399,6 +437,13 @@ Predication (pred) Field Encoding
 Twin-predication (tpred) Field Encoding
 =======================================
 
+Twin-predication (ability to associate two predicate registers with an
+instruction) applies to MV, FCLASS, LD and ST.  The same format also
+applies to integer-branch-compare operations although it is **not** to be
+considered "twin" predication.  In the case of integer-branch-compare
+operations, the second register (if enabled) stores the results of the
+element comparisons.  See Appendix_ for details.
+
 +-------+------------+--------------------+----------------------------------------------+
 | tpred | Mnemonic   | Predicate Register | Meaning                                      |
 +=======+============+====================+==============================================+
@@ -595,7 +640,7 @@ However if partial functionality is implemented, the unimplemented bits
 in STATE and SVPSTATE must be zero, and, in the UNIX Platform, an illegal exception
 **MUST** be raised if unsupported bits are written to.
 
-SVPSTATE fields are exactly the sane layout as STATE:
+SVPSTATE fields are exactly the same layout as STATE:
 
 +----------+----------+----------+----------+----------+---------+---------+
 | (31..28) | (27..26) | (25..24) | (23..18) | (17..12) | (11..6) | (5...0) |
@@ -603,6 +648,8 @@ SVPSTATE fields are exactly the sane layout as STATE:
 | rsvd     | dsvoffs  | subvl    | destoffs | srcoffs  | vl      | maxvl   |
 +----------+----------+----------+----------+----------+---------+---------+
 
+However note that where STATE stores the scalar register number to be used as VL, SVPSTATE.VL actually contains the actual VL value, in an identical fashion to RVV.
+
 Additional Instructions
 =======================