From 10bafd848508228d9fa7b38a58fb752771c141d2 Mon Sep 17 00:00:00 2001
From: Wesley Moore <wes@wezm.net>
Date: Fri, 14 Feb 2025 21:09:46 +1000
Subject: [PATCH] Add multiplication implementation
---
Makefile | 5 ++++-
math.s | 26 ++++++++++++++++++++++++++
tests/test_math.sh | 21 +++++++++++++++++++--
3 files changed, 49 insertions(+), 3 deletions(-)
diff --git a/Makefile b/Makefile
index 94f754b..b610fcd 100644
--- a/Makefile
+++ b/Makefile
@@ -20,11 +20,14 @@ hello.elf: hello.o
calc.elf: mem.o hex.o debug.o math.o calc.o
$(LD) -m elf32lriscv -T link.ld $^ -o $@
-tests: tests/btohex.elf tests/tohex.elf tests/math_add64.elf
+tests: tests/btohex.elf tests/tohex.elf tests/math_add64.elf tests/math_mul.elf
tests/math_add64.elf: hex.o math.o tests/math_add64.o
$(LD) -m elf32lriscv -T link.ld $^ -o $@
+tests/math_mul.elf: hex.o math.o tests/math_mul.o
+ $(LD) -m elf32lriscv -T link.ld $^ -o $@
+
tests/btohex.elf: mem.o hex.o debug.o tests/btohex.o
$(LD) -m elf32lriscv -T link.ld $^ -o $@
diff --git a/math.s b/math.s
index 2389918..9e2b2d1 100644
--- a/math.s
+++ b/math.s
@@ -1,4 +1,5 @@
.globl add64
+.globl mul
.text
@@ -24,3 +25,28 @@ add64:
add a1, t0, t1 # upper 32 bits of answer (upper sum + carry bit)
ret
+# 32-bit shift-add multiplication
+# arguments:
+# a0: multiplicand
+# a1: multiplier
+# return:
+# a0 = a0 × a1
+#
+mul:
+ mv t0, a1 # Save multiplier in t0
+ li a1, 0 # Initialize product in a1
+
+multiply_loop:
+ beqz t0, done # If multiplier is 0, we're done
+ andi t1, t0, 1 # Check least significant bit
+ beqz t1, shift # If LSB is 0, skip addition
+ add a1, a1, a0 # Add multiplicand to product
+
+shift:
+ slli a0, a0, 1 # Shift multiplicand left
+ srli t0, t0, 1 # Shift multiplier right
+ j multiply_loop # Continue loop
+
+done:
+ mv a0, a1 # Move product to return register
+ ret
diff --git a/tests/test_math.sh b/tests/test_math.sh
index 56020e4..9897d7c 100644
--- a/tests/test_math.sh
+++ b/tests/test_math.sh
@@ -2,8 +2,10 @@
test_add64() {
result=$("${QEMU}" -B 0x80000000 -s 2k tests/math_add64.elf)
- # 3.5
- # 3.75
+ # 3.5
+ # 3.75
+ # 1.25
+ # -1.25
expected=$(cat << END
00000003.80000000
00000003.C0000000
@@ -14,3 +16,18 @@ END
# different inputs.
test $? -eq 0 && test "$result" = "$expected"
}
+
+test_mul() {
+ result=$("${QEMU}" -B 0x80000000 -s 2k tests/math_mul.elf)
+ expected=$(cat << END
+0000000F
+0000003F
+0063FF9C
+FFFFFFEB
+END
+)
+
+ # TODO: Ideally this test would allow calling the binary repeatedly with
+ # different inputs.
+ test $? -eq 0 && test "$result" = "$expected"
+}