Add Problem 18.5 exercise

examples/exercises/18p5/Makefile

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+TARGET = gf_inv
+
+$(TARGET).objdump: $(TARGET)
+	riscv64-unknown-elf-objdump -Dls $(TARGET) > $(TARGET).objdump
+    
+$(TARGET): $(TARGET).S Makefile
+	riscv64-unknown-elf-gcc -g -o $(TARGET) -march=rv32gc -mabi=ilp32 -mcmodel=medany \
+	    -nostartfiles -nostdlib -T../../link/link.ld $(TARGET).S
+
+sim:
+	spike --isa=rv32gc_zicsr_zicntr +signature=$(TARGET).signature.output +signature-granularity=4 $(TARGET)
+	diff --ignore-case $(TARGET).signature.output $(TARGET).reference_output || exit
+	echo "Signature matches! Success!"
+
+clean:
+	rm -f $(TARGET) $(TARGET).objdump $(TARGET).signature.output
+	rm -f *~
+
+
+
+

examples/exercises/18p5/README.md

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+Extended Euclidean Algorithm
+
+EEA Cycle Estimates
+
+Main loop body: 25
+swap_and_negate: 22
+gf_degree: 10
+
+Pseuo-Code
+
+r0 := m
+r1 := a
+s0 := 0
+s1 := 1
+
+while r1 \neq 0:
+    deg_r0 = degree(r0)
+    deg_r1 = degree(r1)
+    shift = deg_r0 - deg_r1
+
+    if shift < 0:
+        swap(r0, r1)
+        swap(s0, s1)
+        shift := -shift
+
+    r0 = r0 ^ (r1 << shift)
+    s0 = s0 ^ (s1 << shift)
+
+if r0 \neq 1:
+    return "No inverse"
+else:
+    return s0
+
+
+LaTeX
+------
+\documentclass{article}
+\usepackage{algorithm}
+\usepackage{algpseudocode}
+\usepackage{amsmath}
+
+\begin{document}
+
+\begin{algorithm}
+\caption{Extended Euclidean Algorithm in GF(2\textsuperscript{8})}
+\begin{algorithmic}[1]
+\Require Input value $a$, modulus $m$ (usually $0x11B$)
+\Ensure Return $s_0$ such that $s_0 \cdot a \equiv 1 \pmod{m}$ if inverse exists
+
+\State $r_0 \gets m$
+\State $r_1 \gets a$
+\State $s_0 \gets 0$
+\State $s_1 \gets 1$
+
+\While{$r_1 \ne 0$}
+    \State $d_0 \gets \text{deg}(r_0)$
+    \State $d_1 \gets \text{deg}(r_1)$
+    \State $\text{shift} \gets d_0 - d_1$
+    
+    \If{$\text{shift} < 0$}
+        \State Swap $r_0 \leftrightarrow r_1$
+        \State Swap $s_0 \leftrightarrow s_1$
+        \State $\text{shift} \gets -\text{shift}$
+    \EndIf
+    
+    \State $r_0 \gets r_0 \oplus (r_1 \ll \text{shift})$
+    \State $s_0 \gets s_0 \oplus (s_1 \ll \text{shift})$
+    
+    \State Mask $r_0$ and $s_0$ to 9 bits: $r_0 \gets r_0 \mathbin{\&} 0x1FF$, $s_0 \gets s_0 \mathbin{\&} 0x1FF$
+\EndWhile
+
+\If{$r_0 = 1$}
+    \State \Return $s_0$ \Comment{Inverse found}
+\Else
+    \State \Return \textbf{error} \Comment{No inverse exists}
+\EndIf
+\end{algorithmic}
+\end{algorithm}
+
+\end{document}

examples/exercises/18p5/gf_inv.S

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+// gf_inv.S
+// james.stine@okstate.edu 9 April 2025
+	
+.text
+	
+.global rvtest_entry_point
+rvtest_entry_point:
+	
+    li a0, 0x32        # Input value to invert in GF(2^8)
+    li t0, 0x11B       # Modulus: irreducible poly x^8 + x^4 + x^3 + x + 1
+
+    # Initialize Extended Euclidean variables: a*x+b*y=gcd(a,b)
+    li t1, 0           # s0 = 0 (old x)
+    li t2, 1           # s1 = 1 (new x)
+    mv t3, t0          # r0 = modulus
+    mv t4, a0          # r1 = input value
+
+    csrr s8, instret   # count instructions at beginning
+
+    # --- Register usage ---
+    # a0: Input / Final result (return value)
+    # t0: Modulus (0x11B)
+    # t1: s0 (previous x value)
+    # t2: s1 (current x value)
+    # t3: r0 (previous remainder)
+    # t4: r1 (current remainder)
+    # t5: degree(r0)
+    # t6: degree(r1)
+    # a1: argument to gf_degree
+    # a2: shift amount
+    # a3: temporary for SLT result
+    # a4/a5: shift results, temporaries
+    # s8: initial seed of instruction count (instret)
+    # s9: final value of instruction count (instret)	
+	
+# --------------------------------------
+# Main loop: Extended Euclidean Division
+# --------------------------------------
+inv_loop:
+    beqz t4, fail          	# If r1 == 0, input is not invertible → fail
+
+    # Compute degree of r0
+    mv a1, t3              	# Set a1 = r0 = (modulus m)
+    call gf_degree         	# Compute deg(r0)
+    mv t5, a0			# degree(r0)
+
+    # Compute degree of r1
+    mv a1, t4              	# Set a1 = r1 = (input a)
+    call gf_degree         	# Compute deg(r1)
+    mv t6, a0			# degree (r1)
+
+    # Compute shift = deg(r0) - deg(r1)
+    sub a2, t5, t6		# alignment r1 with highest term in r0
+    slt a3, a2, zero       	# Check if shift < 0
+    bnez a3, swap_and_negate
+
+    # Perform r0 ^= r1 << shift 
+    # Polynomial subtraction in GF(2)
+    sll a4, t4, a2
+    xor t3, t3, a4
+    andi t3, t3, 0x1FF     	# Mask off 9 bits to stay in GF(2^8)
+
+    # Update s0: s0 ^= s1 << shift - Update Bezout coefficient
+    sll a5, t2, a2		
+    xor t1, t1, a5
+    andi t1, t1, 0x1FF		# Mask off 9 bits to stay in GF(2^8)
+
+    # Swap (r0, r1)
+    mv a4, t3
+    mv t3, t4
+    mv t4, a4
+
+    # Swap (s0, s1)
+    mv a4, t1
+    mv t1, t2
+    mv t2, a4
+
+    # Check if r0 == 1, then we are done
+    li t5, 1
+    beq t3, t5, done
+
+    j inv_loop
+
+# -------------------------------------------
+# Case: shift < 0 → negate, then apply shift
+# -------------------------------------------
+swap_and_negate:
+    # Swap r0 <-> r1
+    mv a4, t3
+    mv t3, t4
+    mv t4, a4
+
+    # Swap s0 <-> s1
+    mv a4, t1
+    mv t1, t2
+    mv t2, a4
+
+    # shift = -shift
+    sub a2, zero, a2
+
+    # r0 ^= r1 << shift
+    sll a4, t4, a2
+    xor t3, t3, a4
+    andi t3, t3, 0x1FF		# Mask off to 9 bits again
+
+    # s0 ^= s1 << shift
+    sll a5, t2, a2
+    xor t1, t1, a5
+    andi t1, t1, 0x1FF
+
+    # Final swap to maintain invariant
+    mv a4, t3
+    mv t3, t4
+    mv t4, a4
+
+    mv a4, t1
+    mv t1, t2
+    mv t2, a4
+
+    # Check if r0 == 1
+    li t5, 1
+    beq t3, t5, done
+
+    j inv_loop
+
+# -------------------------------------
+# Helper: compute degree of a1 (MSB set)
+# -------------------------------------
+gf_degree:
+    li t0, 8              	# Start checking from MSB down
+deg_loop:
+    srl a0, a1, t0        	# Right shift a1 by t0
+    andi a0, a0, 1        	# Isolate LSB
+    bnez a0, done_deg     	# If bit is set, we're done
+    addi t0, t0, -1
+    bgez t0, deg_loop     	# Check next bit
+    li a0, 0              	# Nothing set
+    ret
+done_deg:
+    mv a0, t0
+    ret
+
+# ---------------------
+# Finalize inverse
+# ---------------------
+done:
+    mv a0, t1             	# Result is a0
+    andi a0, a0, 0xFF     	# Mask to 8 bits
+    csrr s9, instret		# count instructions at end	
+    sub s9, s9, s8	        # get number of #instructions executed	
+
+write_tohost:			# HTIF stuff
+    la t1, tohost
+    li t0, 1            	# 1 for success, 3 for failure
+    sw t0, 0(t1)        	# send success code
+    
+    la t0, begin_signature	# Address of signature
+    sw a0, 0(t0)            	# Store result (i.e., a0)
+    sw s9, 4(t0)            	# record #instructions executed
+    
+fail:
+    li a0, 0xff			# Signal failure: no inverse exists
+	
+self_loop:
+    j self_loop
+
+.section .tohost 
+tohost:                 	# write to HTIF
+    .word 0
+fromhost:
+    .word 0
+
+.data
+.EQU XLEN,32
+begin_signature:
+    .fill 2*(XLEN/32),4,0xdeadbeef     
+end_signature:
+
+	
+.bss
+    .space 512
+

examples/exercises/18p5/gf_inv.reference_output

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+00000092
+00000398