diff --git a/hw/rtl/VX_csr_unit.v b/hw/rtl/VX_csr_unit.v
index 61755266..1689121a 100644
--- a/hw/rtl/VX_csr_unit.v
+++ b/hw/rtl/VX_csr_unit.v
@@ -41,7 +41,9 @@ module VX_csr_unit #(
     wire csr_we_s1;
     wire [`CSR_ADDR_BITS-1:0] csr_addr_s1;    
     wire [31:0] csr_read_data, csr_read_data_s1;
-    wire [31:0] csr_updated_data_s1;    
+    wire [31:0] csr_updated_data_s1;  
+
+    wire write_enable = csr_pipe_rsp_if.valid && csr_we_s1;
 
     VX_csr_data #(
         .CORE_ID(CORE_ID)
@@ -54,7 +56,7 @@ module VX_csr_unit #(
         .read_addr      (csr_pipe_req_if.csr_addr),
         .read_wid       (csr_pipe_req_if.wid),      
         .read_data      (csr_read_data),
-        .write_enable   (csr_we_s1),        
+        .write_enable   (write_enable),        
         .write_addr     (csr_addr_s1), 
         .write_wid      (csr_pipe_rsp_if.wid),
         .write_data     (csr_updated_data_s1[`CSR_WIDTH-1:0]),
@@ -89,21 +91,22 @@ module VX_csr_unit #(
             default: csr_updated_data = 32'hdeadbeef;
         endcase
     end
-    
-    wire csr_we_s0 = csr_we_s0_unqual && csr_pipe_req_if.valid;
 
-    wire stall = (~csr_pipe_rsp_if.ready && csr_pipe_rsp_if.valid) 
-              || fpu_pending[csr_pipe_req_if.wid];
+    wire stall_in = fpu_pending[csr_pipe_req_if.wid];
+
+    wire pipe_req_valid_qual = csr_pipe_req_if.valid && !stall_in;  
+
+    wire stall_out = ~csr_pipe_rsp_if.ready && csr_pipe_rsp_if.valid;
 
     VX_generic_register #(
         .N(1 + `NW_BITS + `NUM_THREADS + 32 + `NR_BITS + 1 + 1 + `CSR_ADDR_BITS + 1 + 32 + 32)
     ) pipe_reg (
         .clk   (clk),
         .reset (reset),
-        .stall (stall),
+        .stall (stall_out),
         .flush (1'b0),
-        .in    ({csr_pipe_req_if.valid, csr_pipe_req_if.wid, csr_pipe_req_if.tmask, csr_pipe_req_if.PC, csr_pipe_req_if.rd, csr_pipe_req_if.wb, csr_we_s0, csr_pipe_req_if.csr_addr, csr_pipe_req_if.is_io, csr_read_data_qual, csr_updated_data}),
-        .out   ({csr_pipe_rsp_if.valid, csr_pipe_rsp_if.wid, csr_pipe_rsp_if.tmask, csr_pipe_rsp_if.PC, csr_pipe_rsp_if.rd, csr_pipe_rsp_if.wb, csr_we_s1, csr_addr_s1,              select_io_rsp,         csr_read_data_s1,   csr_updated_data_s1})
+        .in    ({pipe_req_valid_qual,   csr_pipe_req_if.wid, csr_pipe_req_if.tmask, csr_pipe_req_if.PC, csr_pipe_req_if.rd, csr_pipe_req_if.wb, csr_we_s0_unqual, csr_pipe_req_if.csr_addr, csr_pipe_req_if.is_io, csr_read_data_qual, csr_updated_data}),
+        .out   ({csr_pipe_rsp_if.valid, csr_pipe_rsp_if.wid, csr_pipe_rsp_if.tmask, csr_pipe_rsp_if.PC, csr_pipe_rsp_if.rd, csr_pipe_rsp_if.wb, csr_we_s1,        csr_addr_s1,              select_io_rsp,         csr_read_data_s1,   csr_updated_data_s1})
     );
 
     for (genvar i = 0; i < `NUM_THREADS; i++) begin
@@ -113,7 +116,7 @@ module VX_csr_unit #(
     end         
 
     // can accept new request?
-    assign csr_pipe_req_if.ready = ~stall;
+    assign csr_pipe_req_if.ready = ~(stall_out || stall_in);
 
     // pending request
     reg [`NUM_WARPS-1:0] pending_r;
diff --git a/hw/rtl/VX_scoreboard.v b/hw/rtl/VX_scoreboard.v
index 6fca200f..58c73b50 100644
--- a/hw/rtl/VX_scoreboard.v
+++ b/hw/rtl/VX_scoreboard.v
@@ -51,7 +51,9 @@ module VX_scoreboard  #(
                 inuse_registers[{ibuf_deq_if.wid, ibuf_deq_if.rd}] <= ibuf_deq_if.tmask;
             end       
             if (release_reg) begin
-                assert(inuse_reg_mask[writeback_if.wid][writeback_if.rd] != 0);
+                assert(inuse_reg_mask[writeback_if.wid][writeback_if.rd] != 0) 
+                    else $error("*** %t: core%0d: invalid writeback register: wid=%0d, PC=%0h, rd=%0d",
+                        $time, CORE_ID, writeback_if.wid, writeback_if.PC, writeback_if.rd);            
                 inuse_registers[{writeback_if.wid, writeback_if.rd}] <= inuse_registers_n;
             end            
             inuse_reg_mask <= inuse_reg_mask_n;
@@ -79,7 +81,7 @@ module VX_scoreboard  #(
             stall_ctr <= 0;
         end else if (ibuf_deq_if.valid && ~ibuf_deq_if.ready) begin            
             stall_ctr <= stall_ctr + 1;
-            assert(stall_ctr < 100000) else $error("%t: core%0d-stalled: wid=%0d, PC=%0h, rd=%0d, wb=%0d, inuse=%b%b%b%b, exe=%b, gpr=%b",
+            assert(stall_ctr < 100000) else $error("*** %t: core%0d-stalled: wid=%0d, PC=%0h, rd=%0d, wb=%0d, inuse=%b%b%b%b, exe=%b, gpr=%b",
                     $time, CORE_ID, ibuf_deq_if.wid, ibuf_deq_if.PC, ibuf_deq_if.rd, ibuf_deq_if.wb, 
                     inuse_regs[ibuf_deq_if.rd], inuse_regs[ibuf_deq_if.rs1], inuse_regs[ibuf_deq_if.rs2], inuse_regs[ibuf_deq_if.rs3], exe_delay, gpr_delay);            
         end else if (ibuf_deq_if.valid && ibuf_deq_if.ready) begin
diff --git a/hw/simulate/simulator.cpp b/hw/simulate/simulator.cpp
index ed22d3a3..45fc9056 100644
--- a/hw/simulate/simulator.cpp
+++ b/hw/simulate/simulator.cpp
@@ -61,6 +61,10 @@ void Simulator::reset() {
   print_bufs_.clear();
   dram_rsp_vec_.clear();
 
+  dram_rsp_active_ = false;
+  snp_req_active_ = false;
+  csr_req_active_ = false;
+
   snp_req_size_ = 0;
   pending_snp_reqs_ = 0;
   csr_rsp_value_ = nullptr;
@@ -91,6 +95,10 @@ void Simulator::step() {
 
   vortex_->clk = 0;
   this->eval();
+
+  dram_rsp_ready_   = vortex_->dram_rsp_ready;
+  snp_req_ready_    = vortex_->snp_req_ready;  
+  csr_io_req_ready_ = vortex_->csr_io_req_ready;
   
   vortex_->clk = 1;
   this->eval();
@@ -132,7 +140,7 @@ void Simulator::eval_dram_bus() {
 
   // send DRAM response  
   if (dram_rsp_active_
-   && vortex_->dram_rsp_valid && vortex_->dram_rsp_ready) {
+   && vortex_->dram_rsp_valid && dram_rsp_ready_) {
     dram_rsp_active_ = false;
   }
   if (!dram_rsp_active_) {
@@ -205,7 +213,7 @@ void Simulator::eval_io_bus() {
 
 void Simulator::eval_snp_bus() {
   if (snp_req_active_) {      
-    if (vortex_->snp_req_valid && vortex_->snp_req_ready) {            
+    if (vortex_->snp_req_valid && snp_req_ready_) {            
       assert(snp_req_size_);
     #ifdef DBG_PRINT_CACHE_SNP
       std::cout << std::dec << timestamp << ": [sim] SNP Req: addr=" << std::hex << vortex_->snp_req_addr << " tag=" << vortex_->snp_req_tag << " remain=" << (snp_req_size_-1) << std::endl;
@@ -238,7 +246,7 @@ void Simulator::eval_snp_bus() {
 
 void Simulator::eval_csr_bus() {
   if (csr_req_active_) { 
-    if (vortex_->csr_io_req_valid && vortex_->csr_io_req_ready) {
+    if (vortex_->csr_io_req_valid && csr_io_req_ready_) {
     #ifndef NDEBUG
       if (vortex_->csr_io_req_rw)
         std::cout << std::dec << timestamp << ": [sim] CSR Wr Req: core=" << (int)vortex_->csr_io_req_coreid << ", addr=" << std::hex << vortex_->csr_io_req_addr << ", value=" << vortex_->csr_io_req_data << std::endl;
diff --git a/hw/simulate/simulator.h b/hw/simulate/simulator.h
index 9330c265..a1613d98 100644
--- a/hw/simulate/simulator.h
+++ b/hw/simulate/simulator.h
@@ -66,6 +66,10 @@ private:
   std::list<dram_req_t> dram_rsp_vec_;
   bool dram_rsp_active_;
 
+  bool dram_rsp_ready_;  
+  bool snp_req_ready_;
+  bool csr_io_req_ready_;
+
   bool snp_req_active_;
   bool csr_req_active_;