diff --git a/e6502.c b/e6502.c
index e9ebed0..d73caa7 100644
--- a/e6502.c
+++ b/e6502.c
@@ -136,11 +136,11 @@ static inline e6502_Address adr_reg_y (e6502_Core* core) {
 }
 
 static inline e6502_Address adr_reg_s (e6502_Core* core) {
-    return &core->registers.S;
+    return &core->registers.P;
 }
 
 static inline e6502_Address adr_reg_sp (e6502_Core* core) {
-    return &core->registers.SP;
+    return &core->registers.S;
 }
 
 // Instructions
@@ -151,53 +151,53 @@ static inline void nop(e6502_Core*, e6502_Address) {
 
 static inline void brk(e6502_Core *core, e6502_Address) {
     e6502_push16(core, core->registers.PC + 1);
-    e6502_push8(core, core->registers.S |
+    e6502_push8(core, core->registers.P |
                       e6502_Status_1 |
                       e6502_Status_B);
-    core->registers.S |= e6502_Status_I;
+    core->registers.P |= e6502_Status_I;
     core->registers.PC = e6502_r16(core, e6502_IRQ_Vec);
 }
 
 static inline void rti(e6502_Core *core, e6502_Address) {
-    core->registers.S = e6502_pop8(core) & ~(e6502_Status_B);
+    core->registers.P = e6502_pop8(core) & ~(e6502_Status_B);
     core->registers.PC = e6502_pop16(core);
 }
 
 static inline void ora(e6502_Core *core, e6502_Address adr) {
     core->registers.A |= e6502_adr_r8(core, adr);
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    core->registers.S |= (core->registers.A & e6502_Status_N);
-    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    core->registers.P |= (core->registers.A & e6502_Status_N);
+    if (!core->registers.A) core->registers.P |= e6502_Status_Z;
 }
 
 static inline void eor(e6502_Core *core, e6502_Address adr) {
     core->registers.A ^= e6502_adr_r8(core, adr);
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    core->registers.S |= (core->registers.A & e6502_Status_N);
-    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    core->registers.P |= (core->registers.A & e6502_Status_N);
+    if (!core->registers.A) core->registers.P |= e6502_Status_Z;
 }
 
 static inline void asl(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr);
-    core->registers.S &= ~(e6502_Status_C |
+    core->registers.P &= ~(e6502_Status_C |
                            e6502_Status_Z |
                            e6502_Status_N);
-    if (val & 0x80) core->registers.S |= e6502_Status_C;
+    if (val & 0x80) core->registers.P |= e6502_Status_C;
     val <<= 1;
-    core->registers.S |= (val & e6502_Status_N);
-    if (!val) core->registers.S |= e6502_Status_Z;
+    core->registers.P |= (val & e6502_Status_N);
+    if (!val) core->registers.P |= e6502_Status_Z;
     e6502_adr_w8(core, adr, val);
 }
 
 static inline void lsr(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr);
-    core->registers.S &= ~(e6502_Status_C |
+    core->registers.P &= ~(e6502_Status_C |
                            e6502_Status_Z |
                            e6502_Status_N);
-    core->registers.S |= (val & e6502_Status_C);
+    core->registers.P |= (val & e6502_Status_C);
     val >>= 1;
-    core->registers.S |= (val & e6502_Status_N);
-    if (!val) core->registers.S |= e6502_Status_Z;
+    core->registers.P |= (val & e6502_Status_N);
+    if (!val) core->registers.P |= e6502_Status_Z;
     e6502_adr_w8(core, adr, val);
 }
 
@@ -205,13 +205,13 @@ static inline void rol(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr);
     uint8_t new_c = (val & 0x80) ? e6502_Status_C : 0;
     val <<= 1;
-    val |= (core->registers.S & e6502_Status_C);
-    core->registers.S &= ~(e6502_Status_C |
+    val |= (core->registers.P & e6502_Status_C);
+    core->registers.P &= ~(e6502_Status_C |
                            e6502_Status_Z |
                            e6502_Status_N);
-    core->registers.S |= new_c;
-    if (!val) core->registers.S |= e6502_Status_Z;
-    core->registers.S |= (val & e6502_Status_N);
+    core->registers.P |= new_c;
+    if (!val) core->registers.P |= e6502_Status_Z;
+    core->registers.P |= (val & e6502_Status_N);
     e6502_adr_w8(core, adr, val);
 }
 
@@ -219,13 +219,13 @@ static inline void ror(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr);
     uint8_t new_c = (val & e6502_Status_C);
     val >>= 1;
-    if (core->registers.S & e6502_Status_C) val |= e6502_Status_N;
-    core->registers.S &= ~(e6502_Status_C |
+    if (core->registers.P & e6502_Status_C) val |= e6502_Status_N;
+    core->registers.P &= ~(e6502_Status_C |
                            e6502_Status_Z |
                            e6502_Status_N);
-    core->registers.S |= new_c;
-    if (!val) core->registers.S |= e6502_Status_Z;
-    core->registers.S |= (val & e6502_Status_N);
+    core->registers.P |= new_c;
+    if (!val) core->registers.P |= e6502_Status_Z;
+    core->registers.P |= (val & e6502_Status_N);
     e6502_adr_w8(core, adr, val);
 }
 
@@ -234,20 +234,20 @@ static inline void pha(e6502_Core *core, e6502_Address adr) {
 }
 
 static inline void php(e6502_Core *core, e6502_Address adr) {
-    e6502_push8(core, core->registers.S |
+    e6502_push8(core, core->registers.P |
                       e6502_Status_1 |
                       e6502_Status_B);
 }
 
 static inline void pla(e6502_Core *core, e6502_Address adr) {
     core->registers.A = e6502_pop8(core);
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
-    core->registers.S |= (core->registers.A & e6502_Status_N);
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    if (!core->registers.A) core->registers.P |= e6502_Status_Z;
+    core->registers.P |= (core->registers.A & e6502_Status_N);
 }
 
 static inline void plp(e6502_Core *core, e6502_Address adr) {
-    core->registers.S = (e6502_pop8(core) |
+    core->registers.P = (e6502_pop8(core) |
                          e6502_Status_1 |
                          e6502_Status_B);
 }
@@ -261,79 +261,79 @@ static inline void b(e6502_Core *core, e6502_Address adr) {
 }
 
 static inline void bpl(e6502_Core *core, e6502_Address adr) {
-    if (!(core->registers.S & e6502_Status_N))  {
+    if (!(core->registers.P & e6502_Status_N))  {
         b(core, adr);
     }
 }
 
 static inline void bmi(e6502_Core *core, e6502_Address adr) {
-    if (core->registers.S & e6502_Status_N)  {
+    if (core->registers.P & e6502_Status_N)  {
         b(core, adr);
     }
 }
 
 static inline void bcc(e6502_Core *core, e6502_Address adr) {
-    if (!(core->registers.S & e6502_Status_C))  {
+    if (!(core->registers.P & e6502_Status_C))  {
         b(core, adr);
     }
 }
 
 static inline void bcs(e6502_Core *core, e6502_Address adr) {
-    if (core->registers.S & e6502_Status_C)  {
+    if (core->registers.P & e6502_Status_C)  {
         b(core, adr);
     }
 }
 
 static inline void bvc(e6502_Core *core, e6502_Address adr) {
-    if (!(core->registers.S & e6502_Status_V))  {
+    if (!(core->registers.P & e6502_Status_V))  {
         b(core, adr);
     }
 }
 
 static inline void bvs(e6502_Core *core, e6502_Address adr) {
-    if (core->registers.S & e6502_Status_V)  {
+    if (core->registers.P & e6502_Status_V)  {
         b(core, adr);
     }
 }
 
 static inline void bne(e6502_Core *core, e6502_Address adr) {
-    if (!(core->registers.S & e6502_Status_Z))  {
+    if (!(core->registers.P & e6502_Status_Z))  {
         b(core, adr);
     }
 }
 
 static inline void beq(e6502_Core *core, e6502_Address adr) {
-    if (core->registers.S & e6502_Status_Z)  {
+    if (core->registers.P & e6502_Status_Z)  {
         b(core, adr);
     }
 }
 
 static inline void clc(e6502_Core *core, e6502_Address) {
-    core->registers.S &= ~e6502_Status_C;
+    core->registers.P &= ~e6502_Status_C;
 }
 
 static inline void cli(e6502_Core *core, e6502_Address) {
-    core->registers.S &= ~e6502_Status_I;
+    core->registers.P &= ~e6502_Status_I;
 }
 
 static inline void cld(e6502_Core *core, e6502_Address) {
-    core->registers.S &= ~e6502_Status_D;
+    core->registers.P &= ~e6502_Status_D;
 }
 
 static inline void clv(e6502_Core *core, e6502_Address) {
-    core->registers.S &= ~e6502_Status_V;
+    core->registers.P &= ~e6502_Status_V;
 }
 
 static inline void sec(e6502_Core *core, e6502_Address) {
-    core->registers.S |= e6502_Status_C;
+    core->registers.P |= e6502_Status_C;
 }
 
 static inline void sei(e6502_Core *core, e6502_Address) {
-    core->registers.S |= e6502_Status_I;
+    core->registers.P |= e6502_Status_I;
 }
 
 static inline void sed(e6502_Core *core, e6502_Address) {
-    core->registers.S |= e6502_Status_D;
+    core->registers.P |= e6502_Status_D;
 }
 
 static inline void jsr(e6502_Core *core, e6502_Address adr) {
@@ -351,13 +351,13 @@ static inline void jmp(e6502_Core *core, e6502_Address adr) {
 
 static inline void bit(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr);
-    core->registers.S &= ~(e6502_Status_Z |
+    core->registers.P &= ~(e6502_Status_Z |
                            e6502_Status_V |
                            e6502_Status_N);
-    core->registers.S |= (val & (e6502_Status_V |
+    core->registers.P |= (val & (e6502_Status_V |
                                  e6502_Status_N));
     if (!(val & core->registers.A)) {
-        core->registers.S |= e6502_Status_Z;
+        core->registers.P |= e6502_Status_Z;
     }
 }
 
@@ -366,12 +366,12 @@ static inline void cmp(e6502_Core *core,
                        uint8_t minuend) {
     uint8_t val = e6502_adr_r8(core, adr);
     uint8_t diff = minuend - val;
-    core->registers.S &= ~(e6502_Status_C |
+    core->registers.P &= ~(e6502_Status_C |
                            e6502_Status_Z |
                            e6502_Status_N);
-    if (minuend >= val) core->registers.S |= e6502_Status_C;
-    if (diff == 0) core->registers.S |= e6502_Status_Z;
-    core->registers.S |= (diff & e6502_Status_N);
+    if (minuend >= val) core->registers.P |= e6502_Status_C;
+    if (diff == 0) core->registers.P |= e6502_Status_Z;
+    core->registers.P |= (diff & e6502_Status_N);
 }
 
 static inline void cpa(e6502_Core *core, e6502_Address adr) {
@@ -388,25 +388,25 @@ static inline void cpy(e6502_Core *core, e6502_Address adr) {
 
 static inline void and(e6502_Core *core, e6502_Address adr) {
     core->registers.A &= e6502_adr_r8(core, adr);
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    core->registers.S |= (core->registers.A & e6502_Status_N);
-    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    core->registers.P |= (core->registers.A & e6502_Status_N);
+    if (!core->registers.A) core->registers.P |= e6502_Status_Z;
 }
 
 static inline void adc(e6502_Core *core, e6502_Address adr) {
     uint8_t addend = e6502_adr_r8(core, adr);
     uint32_t sum = core->registers.A +
                    addend +
-                   (core->registers.S & e6502_Status_C);
-    core->registers.S &= ~(e6502_Status_C | e6502_Status_Z |
+                   (core->registers.P & e6502_Status_C);
+    core->registers.P &= ~(e6502_Status_C | e6502_Status_Z |
                            e6502_Status_V | e6502_Status_N);
-    if (sum > 0xFF) core->registers.S |= e6502_Status_C;
-    if (!(uint8_t)sum) core->registers.S |= e6502_Status_Z;
+    if (sum > 0xFF) core->registers.P |= e6502_Status_C;
+    if (!(uint8_t)sum) core->registers.P |= e6502_Status_Z;
     if (    (addend & 0x80) == (core->registers.A & 0x80) &&
             (sum & 0x80) != (addend & 0x80)) {
-        core->registers.S |= e6502_Status_V;
+        core->registers.P |= e6502_Status_V;
     }
-    core->registers.S |= (sum & e6502_Status_N);
+    core->registers.P |= (sum & e6502_Status_N);
     core->registers.A = sum;
 }
 
@@ -414,32 +414,32 @@ static inline void sbc(e6502_Core *core, e6502_Address adr) {
     uint8_t addend = e6502_adr_r8(core, adr);
     uint32_t sum = core->registers.A -
                    addend -
-                   (1 - (core->registers.S & e6502_Status_C));
-    core->registers.S &= ~(e6502_Status_C | e6502_Status_Z |
+                   (1 - (core->registers.P & e6502_Status_C));
+    core->registers.P &= ~(e6502_Status_C | e6502_Status_Z |
                            e6502_Status_V | e6502_Status_N);
-    if (sum < 0x100) core->registers.S |= e6502_Status_C;
-    if (!(uint8_t)sum) core->registers.S |= e6502_Status_Z;
+    if (sum < 0x100) core->registers.P |= e6502_Status_C;
+    if (!(uint8_t)sum) core->registers.P |= e6502_Status_Z;
     if (    (sum & 0x80) != (core->registers.A & 0x80) &&
             (addend & 0x80) != (core->registers.A & 0x80)) {
-        core->registers.S |= e6502_Status_V;
+        core->registers.P |= e6502_Status_V;
     }
-    core->registers.S |= (sum & e6502_Status_N);
+    core->registers.P |= (sum & e6502_Status_N);
     core->registers.A = sum;
 }
 
 static inline void inc(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr) + 1;
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    core->registers.S |= (val & e6502_Status_N);
-    if (!val) core->registers.S |= e6502_Status_Z;
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    core->registers.P |= (val & e6502_Status_N);
+    if (!val) core->registers.P |= e6502_Status_Z;
     e6502_adr_w8(core, adr, val);
 }
 
 static inline void dec(e6502_Core *core, e6502_Address adr) {
     uint8_t val = e6502_adr_r8(core, adr) - 1;
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    core->registers.S |= (val & e6502_Status_N);
-    if (!val) core->registers.S |= e6502_Status_Z;
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    core->registers.P |= (val & e6502_Status_N);
+    if (!val) core->registers.P |= e6502_Status_Z;
     e6502_adr_w8(core, adr, val);
 }
 
@@ -458,9 +458,9 @@ static inline void sty(e6502_Core *core, e6502_Address adr) {
 static inline void t(e6502_Core *core,
                      e6502_Address adr,
                      uint8_t reg) {
-    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
-    core->registers.S |= (reg & e6502_Status_N);
-    if (!reg) core->registers.S |= e6502_Status_Z;
+    core->registers.P &= ~(e6502_Status_Z | e6502_Status_N);
+    core->registers.P |= (reg & e6502_Status_N);
+    if (!reg) core->registers.P |= e6502_Status_Z;
     e6502_adr_w8(core, adr, reg);
 }
 
@@ -469,7 +469,7 @@ static inline void ta(e6502_Core *core, e6502_Address adr) {
 }
 
 static inline void txs(e6502_Core *core, e6502_Address adr) {
-    core->registers.SP = core->registers.X;
+    core->registers.S = core->registers.X;
 }
 
 static inline void txa(e6502_Core *core, e6502_Address adr) {
@@ -481,7 +481,7 @@ static inline void ty(e6502_Core *core, e6502_Address adr) {
 }
 
 static inline void ts(e6502_Core *core, e6502_Address adr) {
-    t(core, adr, core->registers.SP);
+    t(core, adr, core->registers.S);
 }
 
 static inline void lda(e6502_Core *core, e6502_Address adr) {
@@ -779,9 +779,9 @@ int e6502_dump_instr(e6502_Core* core, e6502_Mem_Addr addr,
 
 int e6502_dump_regs(e6502_Core* core, FILE* file) {
     fprintf(file, "S:%02x A:%02x X:%02x Y:%02x P:%02x\n",
-            core->registers.SP, core->registers.A,
+            core->registers.S, core->registers.A,
             core->registers.X, core->registers.Y,
-            core->registers.S);
+            core->registers.P);
 }
 */
 
@@ -837,6 +837,6 @@ int e6502_run(e6502_Core* core,
 
 int e6502_reset(e6502_Core* core) {
     core->registers.PC = e6502_r16(core, e6502_Reset_Vec);
-    core->registers.SP -= 3;
-    core->registers.S |= (e6502_Status_B | e6502_Status_1);
+    core->registers.S -= 3;
+    core->registers.P |= (e6502_Status_B | e6502_Status_1);
 }
diff --git a/e6502.h b/e6502.h
index 568d116..6e388ea 100644
--- a/e6502.h
+++ b/e6502.h
@@ -14,11 +14,11 @@ typedef enum {
 
 typedef struct {
     uint16_t PC;
-    uint8_t  SP;
+    uint8_t  S;
     uint8_t  A;
     uint8_t  X;
     uint8_t  Y;
-    uint8_t  S;
+    uint8_t  P;
 } e6502_Registers;
 
 typedef struct {
@@ -68,7 +68,7 @@ static inline void e6502_w16(e6502_Core* core,
 }
 
 static inline void e6502_push8(e6502_Core* core, uint8_t val) {
-    e6502_w8(core, e6502_Memory_Stack + core->registers.SP--, val);
+    e6502_w8(core, e6502_Memory_Stack + core->registers.S--, val);
 }
 
 static inline void e6502_push16(e6502_Core* core, uint16_t val) {
@@ -78,7 +78,7 @@ static inline void e6502_push16(e6502_Core* core, uint16_t val) {
 
 static inline uint8_t e6502_pop8(e6502_Core* core) {
     return e6502_r8(core, e6502_Memory_Stack +
-                          ++core->registers.SP);
+                          ++core->registers.S);
 }
 
 static inline uint16_t e6502_pop16(e6502_Core* core) {