work on BX

Author: evanwporter

cores/gba/rtl/cpu/CPU.sv

@@ -393,6 +393,13 @@ module ARM7TMDI (
         $display("Restoring CPSR from SPSR_%0d: 0x%08x", cpu_mode, read_spsr(regs, cpu_mode));
       end
 
+      if (control_signals.set_thumb_mode) begin
+        regs.CPSR[5] <= B_bus[0];
+        `WRITE_REG(regs, cpu_mode, 4'd15, B_bus & 32'hFFFFFFFE)
+        flush_req <= 1'b1;
+        $display("Setting Thumb mode bit in CPSR");
+      end
+
       if (control_signals.ALU_set_flags && control_signals.pipeline_advance) begin
 
         if ((decoder_bus.word.Rd == 4'd15) && mode_has_spsr(
@@ -452,7 +459,6 @@ module ARM7TMDI (
           `WRITE_REG(regs, control_signals.force_user_mode ? CPU_MODE_USR : cpu_mode,
                      decoder_bus.word.Rn, alu_bus.result)
         end
-        ALU_WB_REG_14: `WRITE_REG(regs, cpu_mode, 14, alu_bus.result)
         ALU_WB_REG_RP: begin
           `WRITE_REG(regs, control_signals.force_user_mode ? CPU_MODE_USR : cpu_mode,
                      control_signals.Rp_imm, alu_bus.result)

cores/gba/rtl/cpu/ControlUnit.sv

@@ -1016,7 +1016,7 @@ module ControlUnit (
         end
 
         // ============================
-        // Branch & Branch with Link
+        // Branch
         // ============================
 
         ARM_INSTR_BRANCH: begin
@@ -1075,6 +1075,22 @@ module ControlUnit (
           end
         end
 
+        ARM_INSTR_BRANCH_EX: begin
+          if (cycle == 8'd0) begin
+            control_signals.ALU_op = ALU_OP_MOV;
+            control_signals.addr_bus_src = ADDR_SRC_ALU;
+
+            // control_signals.ALU_writeback = ALU_WB_REG_RP;
+            // control_signals.Rp_imm = 4'd15;
+
+            control_signals.B_bus_source = B_BUS_SRC_REG_RN;
+
+            control_signals.set_thumb_mode = 1'b1;
+
+            control_signals.pipeline_advance = 1'b1;
+          end
+        end
+
         ARM_INSTR_SWI: begin
           if (cycle == 8'd0) begin
             // Rd should be R14 (LR)

cores/gba/rtl/cpu/Decoder.sv

@@ -44,6 +44,8 @@ module GBA_Decoder (
     priority casez (IR)
       /// Branch and Branch Exchange
       32'b????_0001_0010_1111_1111_1111_0001_????: begin
+        bus.word.instr_type = ARM_INSTR_BRANCH_EX;
+        bus.word.Rn = IR[3:0];
         $display("[GBA_Decoder] Detected BX instruction with IR=0x%08x", IR);
       end
 

cores/gba/rtl/cpu/control_types.sv

@@ -82,10 +82,7 @@ package gba_control_types_pkg;
     ALU_WB_REG_RN,
 
     /// If set, we need also need to set `Rp_imm`
-    ALU_WB_REG_RP,
-
-    /// Write back to register 14
-    ALU_WB_REG_14
+    ALU_WB_REG_RP
   } alu_writeback_source_t;
 
   typedef struct packed {
@@ -114,6 +111,8 @@ package gba_control_types_pkg;
 
     exception_t exception;
 
+    logic set_thumb_mode;
+
     // ======================================================
     // A Bus
     // ======================================================
@@ -127,8 +126,6 @@ package gba_control_types_pkg;
     // B Bus
     // ======================================================
 
-    /// Hello
-    /// There
     B_bus_source_t B_bus_source;
 
     /// Immediate value to place on the B bus, if selected in `B_bus_source`