From 82cc71261d3d32012d33d3bebe56ca5e3b0bcdbd Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 2 Oct 2022 15:32:49 -0700
Subject: Initial commit.

---
 isa/decode.cpp | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)
 create mode 100644 isa/decode.cpp

(limited to 'isa/decode.cpp')

diff --git a/isa/decode.cpp b/isa/decode.cpp
new file mode 100644
index 0000000..8a85d41
--- /dev/null
+++ b/isa/decode.cpp
@@ -0,0 +1,96 @@
+#include <cassert>
+
+#include "isa/isa.h"
+
+instruction_context decode(unsigned int dfifb, unsigned int pc, unsigned int bits)
+{
+    instruction_context inst;
+
+    auto df = dfifb >> 3;
+    auto ifb = dfifb & 00007;
+
+    inst.next_pc = (pc & ~07777) | ((pc + 1) & 07777);
+
+    switch (bits >> 9) {
+        case 0: // AND
+            inst.need_exec_load = true;
+            inst.need_read_acc = true;
+            inst.need_write_acc = true;
+            inst.ef = [](auto &ctx) {
+                ctx.acc = ctx.acc.value() & ctx.data.value();
+            };
+            break;
+        case 1: // TAD
+            inst.need_exec_load = true;
+            inst.need_read_acc = true;
+            inst.need_read_link = true;
+            inst.need_write_acc = true;
+            inst.need_write_link = true;
+            inst.ef = [](auto &ctx) {
+                unsigned int sum = (ctx.link.value() << 12) + ctx.acc.value() + ctx.data.value();
+                ctx.link = (sum >> 12) & 1;
+                ctx.acc = sum & 07777;
+            };
+            break;
+        case 2: // ISZ
+            inst.need_exec_load = true;
+            inst.need_exec_store = true;
+            inst.possibly_redirects = true;
+            inst.ef = [](auto &ctx) {
+                ctx.data = (ctx.data.value() + 1) & 07777;
+                if (*ctx.data)
+                    ctx.next_pc = (ctx.next_pc & ~07777) | ((ctx.next_pc + 1) & 07777);
+            };
+            break;
+        case 3: // DCA
+            inst.need_read_acc = true;
+            inst.need_write_acc = true;
+            inst.need_exec_store = true;
+            inst.ef = [](auto &ctx) {
+                ctx.data = ctx.acc.value();
+                ctx.acc = 0;
+            };
+            break;
+        case 4: // JMS
+            inst.need_exec_store = true;
+            inst.possibly_redirects = true;
+            inst.ef = [ifb](auto &ctx) {
+                ctx.data = ctx.next_pc;
+                ctx.next_pc = (ifb << 12) | ((ctx.final_address.value() + 1) & 07777);
+            };
+            break;
+        case 5: // JMP
+            inst.possibly_redirects = true;
+            inst.ef = [ifb](auto &ctx) {
+                ctx.next_pc = (ifb << 12) | (ctx.final_address.value() & 07777);
+            };
+            break;
+        case 6: // IOT
+            inst.ef = [bits](auto &ctx) {
+                assert(false);
+            };
+            break;
+        case 7: // OPR
+            inst.ef = [bits](auto &ctx) {
+                assert(false);
+            };
+            break;
+    }
+
+    // Instructions with memory operands may be direct or indirect
+    if (inst.need_exec_load || inst.need_exec_store || inst.possibly_redirects) {
+        auto addr = (df << 12) | ((bits & 00200) ? (next_pc & 07600) : 0) | (bits & 00177);
+        if (bits & 00400) {
+            inst.need_indirect_load = true;
+            inst.init_address = addr;
+        } else {
+            inst.final_address = addr;
+        }
+    }
+
+    // Non-jump indirect memory operands may be autoincrementing depending on operand bits
+    if (!inst.possibly_redirects && inst.need_indirect_load && ((bits & 00170) == 00010))
+        inst.need_autoinc_store = true;
+
+    return inst;
+}
-- 
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