From cffffab8ea0cc65d514e27dfad45ba009c37f49b Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Fri, 21 Oct 2022 05:11:53 -0300
Subject: [PATCH] feat(cpu): refactor LDM/STM

---
 src/cpu/block_data_transfer.zig | 81 ++++++++++++++++++---------------
 1 file changed, 45 insertions(+), 36 deletions(-)

diff --git a/src/cpu/block_data_transfer.zig b/src/cpu/block_data_transfer.zig
index 22600cf..638043f 100644
--- a/src/cpu/block_data_transfer.zig
+++ b/src/cpu/block_data_transfer.zig
@@ -8,48 +8,57 @@ pub fn blockDataTransfer(comptime P: bool, comptime U: bool, comptime S: bool, c
     return struct {
         fn inner(cpu: *Arm7tdmi, bus: *Bus, opcode: u32) void {
             const rn = opcode >> 16 & 0xF;
-            var base = cpu.r[rn];
+            const base = cpu.r[rn];
 
-            // TODO: For Performance (?) if U we got from 0 -> 0xF, if !U, 0xF -> 0
-            // we can do this and have it be fast because of comptime, baby!
-
-            if (!U) {
-                var count: u32 = 0;
-                var i: u5 = 0;
-                while (i < 0x10) : (i += 1) {
-                    count += opcode >> i & 1;
-                }
-
-                base -= count * 4;
-            }
-
-            var address = if (@boolToInt(P) ^ @boolToInt(U) == 0) base + 4 else base;
             if (S and opcode >> 15 & 1 == 0) std.debug.panic("[CPU] TODO: STM/LDM with S set but R15 not in transfer list", .{});
 
-            var i: u5 = 0;
-            while (i < 0x10) : (i += 1) {
-                if (opcode >> i & 1 == 1) {
-                    if (L) {
-                        cpu.r[i] = bus.read32(address);
-                        if (S and i == 0xF) std.debug.panic("[CPU] TODO: SPSR_<mode> is transferred to CPSR", .{});
-                    } else {
-                        if (i == 0xF) {
-                            if (!S) {
-                                // TODO: Assure that this is Address of STM instruction + 12
-                                bus.write32(address, cpu.r[i] + (12 - 4));
-                            } else {
-                                std.debug.panic("[CPU] TODO: STM with S set and R15 in transfer list", .{});
-                            }
-                        } else {
-                            bus.write32(address, cpu.r[i]);
-                        }
-                    }
+            if (U) {
+                // Increment
+                var address = if (P) base + 4 else base;
 
-                    address += 4;
+                var i: u5 = 0;
+                while (i < 0x10) : (i += 1) {
+                    if (opcode >> i & 1 == 1) {
+                        transfer(cpu, bus, i, address);
+                        address += 4;
+                    }
+                }
+
+                if (W and P or !P) cpu.r[rn] = address - 4;
+            } else {
+                // Decrement
+                var address = if (P) base - 4 else base;
+
+                var i: u5 = 0x10;
+                while (i > 0) : (i -= 1) {
+                    const reg_idx = i - 1;
+
+                    if (opcode >> reg_idx & 1 == 1) {
+                        transfer(cpu, bus, reg_idx, address);
+                        address -= 4;
+                    }
+                }
+
+                if (W and P or !P) cpu.r[rn] = address + 4;
+            }
+        }
+
+        fn transfer(cpu: *Arm7tdmi, bus: *Bus, i: u5, address: u32) void {
+            if (L) {
+                cpu.r[i] = bus.read32(address);
+                if (S and i == 0xF) std.debug.panic("[CPU] TODO: SPSR_<mode> is transferred to CPSR", .{});
+            } else {
+                if (i == 0xF) {
+                    if (!S) {
+                        // TODO: Assure that this is Address of STM instruction + 12
+                        bus.write32(address, cpu.r[i] + (12 - 4));
+                    } else {
+                        std.debug.panic("[CPU] TODO: STM with S set and R15 in transfer list", .{});
+                    }
+                } else {
+                    bus.write32(address, cpu.r[i]);
                 }
             }
-
-            if (W and P or !P) cpu.r[rn] = if (U) address - 4 else base;
         }
     }.inner;
 }