From 7a61d1a214059a920e2d53fcae47513f1e2a7936 Mon Sep 17 00:00:00 2001
From: Nathaniel Walizer <robots.are.love@gmail.com>
Date: Sun, 29 Dec 2024 18:47:05 -0800
Subject: [PATCH] Simplify single-step stages

---
 e6502.c | 96 +++++++++++++++++++++++++++++++++------------------------
 e6502.h |  3 +-
 2 files changed, 57 insertions(+), 42 deletions(-)

diff --git a/e6502.c b/e6502.c
index 3ce93b2..28e49bb 100644
--- a/e6502.c
+++ b/e6502.c
@@ -926,17 +926,62 @@ static void e6502_nmi(e6502_Core* core) {
     core->registers.PC = e6502_r16(core, e6502_NMI_Vec);
 }
 
+int e6502_step(e6502_Core* core) {
+    core->cycle = 0;
+
+#ifdef E6502_DEBUG
+    int last_pc = core->registers.PC;
+#endif
+
+    uint8_t opcode = e6502_r8(core, core->registers.PC);
+    const e6502_Instruction* instr =
+            &e6502_instructions[opcode];
+    if (!instr->operator) {
+#ifdef E6502_DEBUG
+        fprintf(stdout, "$%04x: $%02x\n", last_pc, opcode);
+#endif
+        core->cycle = -1;
+
+    } else {
+        core->registers.PC++;
+        e6502_Address adr = instr->address(core);
+#ifdef E6502_DEBUG
+        uint16_t size = core->registers.PC - last_pc;
+        uint16_t val = 0;
+        if (size > 1) {
+            if (size == 2) val = e6502_r8(core, last_pc + 1);
+            else val = e6502_r16(core, last_pc + 1);
+        }
+        e6502_dump_regs(core, stdout);
+        fprintf(stdout, "$%04x: ", last_pc);
+        e6502_fprintf_instr(stdout, opcode, (uint8_t*)&val);
+        fputc('\n', stdout);
+#endif
+        instr->operator(core, adr);
+        core->cycle += instr->cycles;
+
+        if (    (core->pins & e6502_Pin_NMI) &&
+                !(core->pins & e6502_NMI_Serviced)) {
+            core->pins |= e6502_NMI_Serviced;
+            e6502_nmi(core);
+        } else if ( (core->pins & e6502_Pin_IRQ) &&
+                    !(core->registers.P & e6502_Status_I)) {
+            e6502_irq(core);
+        }
+    }
+
+    return core->cycle;
+}
 
 int e6502_run(e6502_Core* core,
               int remaining,
               int* run,
               int instructions) {
     int status = 0;
-    int i_count = 0;
-    uint64_t start = (instructions ? i_count : core->cycle);
-    uint64_t end = start + remaining;
+    int count = 0;
     int last_pc = -1;
-    while ((instructions ? i_count : core->cycle) < end) {
+    (void)last_pc;
+    while (count < remaining) {
 #ifdef E6502_HCF
         if (core->registers.PC == last_pc) {
             // Trapped.
@@ -945,51 +990,20 @@ int e6502_run(e6502_Core* core,
         }
 #endif
         last_pc = core->registers.PC;
-        (void)last_pc;
-        uint8_t opcode = e6502_r8(core, core->registers.PC);
-        const e6502_Instruction* instr =
-                &e6502_instructions[opcode];
-        if (!instr->operator) {
-#ifdef E6502_DEBUG
-            fprintf(stdout, "$%04x: $%02x\n", last_pc, opcode);
-#endif
-            status = -1;
+        int cpu_cycles = e6502_step(core);
+        if (cpu_cycles <= 0) {
+            status = cpu_cycles;
             break;
-        } else {
-            core->registers.PC++;
-            e6502_Address adr = instr->address(core);
-#ifdef E6502_DEBUG
-            uint16_t size = core->registers.PC - last_pc;
-            uint16_t val = 0;
-            if (size > 1) {
-                if (size == 2) val = e6502_r8(core, last_pc + 1);
-                else val = e6502_r16(core, last_pc + 1);
-            }
-            e6502_dump_regs(core, stdout);
-            fprintf(stdout, "$%04x: ", last_pc);
-            e6502_fprintf_instr(stdout, opcode, (uint8_t*)&val);
-            fputc('\n', stdout);
-#endif
-            instr->operator(core, adr);
-            core->cycle += instr->cycles;
-            i_count++;
-
-            if (    (core->pins & e6502_Pin_NMI) &&
-                    !(core->pins & e6502_NMI_Serviced)) {
-                core->pins |= e6502_NMI_Serviced;
-                e6502_nmi(core);
-            } else if ( (core->pins & e6502_Pin_IRQ) &&
-                        !(core->registers.P & e6502_Status_I)) {
-                e6502_irq(core);
-            }
         }
+        count += (instructions ? 1 : cpu_cycles);
     }
     if (run) {
-        *run = (instructions ? i_count : core->cycle) - start;
+        *run = count;
     }
     return status;
 }
 
+
 #ifdef E6502_POLL_MEM
 void e6502_init(e6502_Core* core) { /* nuthin' */ }
 #else
diff --git a/e6502.h b/e6502.h
index ab865b7..98dc001 100644
--- a/e6502.h
+++ b/e6502.h
@@ -45,7 +45,7 @@ typedef struct {
     e6502_Write* bus_write;
     void*        bus_context;
 #endif
-    uint64_t cycle;
+    int cycle;
 } e6502_Core;
 
 #define e6502_Memory_Stack  (0x0100U)
@@ -120,6 +120,7 @@ void e6502_init(e6502_Core*);
 void e6502_init(e6502_Core*, e6502_Read, e6502_Write, void*);
 #endif
 
+int e6502_step(e6502_Core* core);
 int e6502_run(e6502_Core* core,
               int remaining,
               int* run,