1 files changed, 16 insertions, 2 deletions
diff --git a/fmc/rtl/novena_fmc_top.v b/fmc/rtl/novena_fmc_top.v
index 37fdc49..2d640ca 100644
--- a/fmc/rtl/novena_fmc_top.v
+++ b/fmc/rtl/novena_fmc_top.v
@@ -217,6 +217,20 @@ module novena_fmc_top
    // hashes, RNGs and ciphers to different regions (segments) of memory.
    //----------------------------------------------------------------
 
+   // A note on byte-swapping:
+   // STM32 is little-endian, while the register interface here is
+   // big-endian. The software reads and writes 32-bit integer values,
+   // which means transmitting the least significant byte first. Up to
+   // now, we've been doing byte-swapping in software, which is
+   // inefficient, especially for bulk data transfer. So now we're doing
+   // the byte-swapping in hardware.
+
+   wire [31:0] 		tmp_write_data;
+   assign tmp_write_data = {sys_fmc_dout[7:0], sys_fmc_dout[15:8], sys_fmc_dout[23:16], sys_fmc_dout[31:24]};
+
+   wire [31 : 0] 	tmp_read_data;
+   assign sys_fmc_din = {tmp_read_data[7:0], tmp_read_data[15:8], tmp_read_data[23:16], tmp_read_data[31:24]};
+
    core_selector cores
      (
       .sys_clk(sys_clk),
@@ -225,8 +239,8 @@ module novena_fmc_top
       .sys_eim_addr(sys_fmc_addr[16:0]),	// XXX parameterize
       .sys_eim_wr(sys_fmc_wren),
       .sys_eim_rd(sys_fmc_rden),
-      .sys_write_data(sys_fmc_dout),
-      .sys_read_data(sys_fmc_din),
+      .sys_write_data(tmp_write_data),
+      .sys_read_data(tmp_read_data),
 
       .noise(ct_noise)
       );