Add a special script for debugging and make changes Hanna recommended.

README.md

@@ -10,6 +10,8 @@ learning, I want the source code commentary to be as educational as possible.
 
 Special thanks to [Georg Sauthoff](https://gms.tf/riscv-vector.html) whose writing really helped me begin this exercise!
 
+Special thanks also to [Hanna Kruppe](https://github.com/hanna-kruppe) who took the time to read through the source code and counsel me on the RISC-V ISA vector extension.
+
 ## Running
 
 As of this writing, the Vector ISA is not standardized so you will need an
@@ -36,6 +38,10 @@ Execute the `build-caesar.sh` script.
 
 Execute the `run-caesar.sh` script.
 
+## Debugging
+
+Execute the `debug-caesar.sh` script.
+
 ## Comments/Issues
 
 Please submit comments and bug reports using Github Issues. Remember, this

caesar.S

@@ -92,14 +92,19 @@ main:
 	# 4. Set t0 to min(a1, VLEN/SEW).
 	vsetvli	t0, a1, e8, m1
 
-	# load t0 bytes (bytes are SEW bits in this case) of plaintext into v0
+	# At this point t0 and vl are equal. vl is the control register that
+	# tells the vector processor how many elements are in each vector
+	# register.
+
+	# load vl (aka t0) bytes (bytes are SEW bits in this case) of plaintext
+	# into v0
 	vlbu.v	v0, (a4)
-	# load t0 bytes of key into 1th vector. This is safe because there are
-	# 4 bytes of key and t0 is never more than 4.
+	# load vl (aka t0) bytes of key into 1th vector. This is safe because
+	# there are 4 bytes of key and t0 is never more than 4.
 	vlbu.v	v1, (a5)
-	# encrypt t0 bytes
+	# encrypt vl (aka t0) bytes
 	vadd.vv	v0, v0, v1
-	# store t0 bytes of encrypted text into memory
+	# store vl (aka t0) bytes of encrypted text into memory
 	vsb.v	v0, (a3)
 
 	# plaintext -= slurped_amt

debug-caesar.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+source directories.sh
+
+# Debug the Caesar Cipher application using Spike and PK
+$SPIKE_INSTALL_DIR/bin/spike -d --isa=RV64gcV $PK_INSTALL_DIR/riscv64-unknown-elf/bin/pk caesar.out
+
+# Debug Spike as it emulates PK running the Caesar Cipher application
+#gdb --args $SPIKE_INSTALL_DIR/bin/spike -d --isa=RV64gcV $PK_INSTALL_DIR/riscv64-unknown-elf/bin/pk caesar.out

run-caesar.sh

@@ -4,9 +4,3 @@ source directories.sh
 
 # Emulate the Caesar Cipher application using Spike and PK
 $SPIKE_INSTALL_DIR/bin/spike --isa=RV64gcV $PK_INSTALL_DIR/riscv64-unknown-elf/bin/pk caesar.out
-
-# Debug the Caesar Cipher application using Spike and PK
-#$SPIKE_INSTALL_DIR/bin/spike -d --isa=RV64gcV $PK_INSTALL_DIR/riscv64-unknown-elf/bin/pk caesar.out
-
-# Debug Spike as it emulates PK running the Caesar Cipher application
-#gdb --args $SPIKE_INSTALL_DIR/bin/spike -d --isa=RV64gcV $PK_INSTALL_DIR/riscv64-unknown-elf/bin/pk caesar.out