From 3b6e25a9b36e938d12595212cb13fd681d33d33d Mon Sep 17 00:00:00 2001
From: Cassie Jones <code@witchoflight.com>
Date: Fri, 10 Jan 2020 21:44:15 -0500
Subject: [PATCH] Add a revision of the ISA design

---
 .gitignore |  2 ++
 isa.txt    | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 isa.txt

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..be30245
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+\#*#
+*.swp
diff --git a/isa.txt b/isa.txt
new file mode 100644
index 0000000..4c77802
--- /dev/null
+++ b/isa.txt
@@ -0,0 +1,43 @@
+ALU2: add sub and lsl rol or xor bic
+ALU1: inc dec neg compl
+ALUI: add7 sub6 lsl lsr asr rot and or xor
+
+76543210
+--------
+0000aabb          => MOVE: a = b   # when a != b
+0000aabb          => CALL: call a  # when a == b, r0 <- ret
+0001aaoo          => ALU1: a = op a
+001faa00 0iiiiiii => LDST: a <-> [imm]
+001faa00 1iiiiibb => LDST: a <-> [b + imm]
+001faa10          => LDST: a <-> [r2]
+001faabb          => LDST: a <-> [b-1:b]
+0100aabb iiiiiiii => JMPR: jump aa  # when a == b
+01010000 iiiiiiii => JMPI: jump offset
+01010101 iiiiiiii => CALI: call offset
+01010101 iiiiiiii => CALI: call offset
+010oaabb oiiiiiii => BRNC: branch if a op b
+0110aa00 0iiiiiii => ALUI: a = a + imm
+0110aa00 10iiiiii => ALUI: a = a - imm
+0110aa00 11oooiii => ALUI: a = a op imm
+0110aaoo iiiiiiii => ALUI: a = a {and, or, xor} imm
+0111aabb          => SKIP: skip if a ule b
+1oooaabb          => ALU2: a = a op b
+
+
+Encoding branch conditions:
+We want: eq/ne, test/testn, lt/gt, ule/ugt
+We have 2 operand bits, so we can encode eq, test, lt, ult, and get the negations by swapping operands.
+For example, if aa < bb, then we use eq, and if bb < aa, we use ne.
+This is natural for lt/gt, and the comparison for eq/ne etc. just needs 1 LUT4.
+
+Load/store should use register pairs, and then specific registers numbers can encode whether it should use the larger instructions.
+
+Skip is valuable because it lets you implement wider add/sub with a relatively short instruction sequence.
+For instance, you can do:
+
+a1 = a1 + b1
+a2 = a2 + b2
+skip if b1 ule a1
+a2 = inc a2
+
+to compute [a2:a1] + [b2:b1]
-- 
2.47.0