diff --git a/Source/control.cpp b/Source/control.cpp
index dfd17db13..24582d01e 100644
--- a/Source/control.cpp
+++ b/Source/control.cpp
@@ -675,7 +675,7 @@ void ToggleSpell(int slot)
 		break;
 	}
 
-	if (spells & (__int64)1 << (plr[myplr]._pSplHotKey[slot] - 1)) {
+	if (spells & SPELLBIT(plr[myplr]._pSplHotKey[slot])) {
 		plr[myplr]._pRSpell = plr[myplr]._pSplHotKey[slot];
 		plr[myplr]._pRSplType = plr[myplr]._pSplTHotKey[slot];
 		force_redraw = 255;
@@ -2462,13 +2462,13 @@ char GetSBookTrans(int ii, BOOL townok)
 		return RSPLTYPE_SKILL;
 #endif
 	st = RSPLTYPE_SPELL;
-	if (plr[myplr]._pISpells & (__int64)1 << (ii - 1)) {
+	if (plr[myplr]._pISpells & SPELLBIT(ii)) {
 		st = RSPLTYPE_CHARGES;
 	}
 #ifdef HELLFIRE
-	if (plr[myplr]._pAblSpells & (__int64)1 << (ii - 1)) {
+	if (plr[myplr]._pAblSpells & SPELLBIT(ii)) {
 #else
-	if (plr[myplr]._pAblSpells & 1 << (ii - 1)) { /// BUGFIX: missing (__int64)
+	if (plr[myplr]._pAblSpells & 1 << (ii - 1)) { /// BUGFIX: missing (__int64) - use SPELLBIT(ii) macro
 #endif
 		st = RSPLTYPE_SKILL;
 	}
@@ -2506,7 +2506,7 @@ void DrawSpellBook()
 	yp = 55 + SCREEN_Y;
 	for (i = 1; i < 8; i++) {
 		sn = SpellPages[sbooktab][i - 1];
-		if (sn != -1 && spl & (__int64)1 << (sn - 1)) {
+		if (sn != -1 && spl & SPELLBIT(sn)) {
 			st = GetSBookTrans(sn, TRUE);
 			SetSpellTrans(st);
 			DrawSpellCel(RIGHT_PANEL_X + 11, yp, pSBkIconCels, SpellITbl[sn], 37);
@@ -2560,12 +2560,12 @@ void CheckSBook()
 	if (MouseX >= RIGHT_PANEL + 11 && MouseX < RIGHT_PANEL + 48 && MouseY >= 18 && MouseY < 314) {
 		sn = SpellPages[sbooktab][(MouseY - 18) / 43];
 		spl = plr[myplr]._pMemSpells | plr[myplr]._pISpells | plr[myplr]._pAblSpells;
-		if (sn != -1 && spl & (__int64)1 << (sn - 1)) {
+		if (sn != -1 && spl & SPELLBIT(sn)) {
 			st = RSPLTYPE_SPELL;
-			if (plr[myplr]._pISpells & (__int64)1 << (sn - 1)) {
+			if (plr[myplr]._pISpells & SPELLBIT(sn)) {
 				st = RSPLTYPE_CHARGES;
 			}
-			if (plr[myplr]._pAblSpells & (__int64)1 << (sn - 1)) {
+			if (plr[myplr]._pAblSpells & SPELLBIT(sn)) {
 				st = RSPLTYPE_SKILL;
 			}
 			plr[myplr]._pRSpell = sn;
diff --git a/Source/debug.cpp b/Source/debug.cpp
index 3dda266b6..feb910a7c 100644
--- a/Source/debug.cpp
+++ b/Source/debug.cpp
@@ -160,7 +160,7 @@ void MaxSpellsCheat()
 
 	for (i = 1; i < MAX_SPELLS; i++) {
 		if (spelldata[i].sBookLvl != -1) {
-			plr[myplr]._pMemSpells |= (__int64)1 << (i - 1);
+			plr[myplr]._pMemSpells |= SPELLBIT(i);
 			plr[myplr]._pSplLvl[i] = 10;
 		}
 	}
@@ -168,7 +168,7 @@ void MaxSpellsCheat()
 
 void SetSpellLevelCheat(char spl, int spllvl)
 {
-	plr[myplr]._pMemSpells |= (__int64)1 << (spl - 1);
+	plr[myplr]._pMemSpells |= SPELLBIT(spl);
 	plr[myplr]._pSplLvl[spl] = spllvl;
 }
 
diff --git a/Source/inv.cpp b/Source/inv.cpp
index da20fae26..d1f938bdf 100644
--- a/Source/inv.cpp
+++ b/Source/inv.cpp
@@ -1697,6 +1697,7 @@ void RemoveInvItem(int pnum, int iv)
 		if (plr[pnum]._pRSpell != SPL_INVALID) {
 			// BUGFIX: Cast the literal `1` to `unsigned __int64` to make that bitshift 64bit
 			// this causes the last 4 skills to not reset correctly after use
+			// replace with SPELLBIT(plr[pnum]._pRSpell)
 			if (!(
 			        plr[pnum]._pScrlSpells
 			        & (1 << (plr[pnum]._pRSpell - 1)))) {
@@ -1761,6 +1762,7 @@ void RemoveSpdBarItem(int pnum, int iv)
 		if (plr[pnum]._pRSpell != SPL_INVALID) {
 			// BUGFIX: Cast the literal `1` to `unsigned __int64` to make that bitshift 64bit
 			// this causes the last 4 skills to not reset correctly after use
+			// replace with SPELLBIT(plr[pnum]._pRSpell)
 			if (!(
 			        plr[pnum]._pScrlSpells
 			        & (1 << (plr[pnum]._pRSpell - 1)))) {
diff --git a/Source/items.cpp b/Source/items.cpp
index 056f0aa9b..e6fa18e74 100644
--- a/Source/items.cpp
+++ b/Source/items.cpp
@@ -727,7 +727,7 @@ void CalcPlrItemVals(int p, BOOL Loadgfx)
 			maxd += itm->_iMaxDam;
 
 			if (itm->_iSpell != SPL_NULL) {
-				spl |= (unsigned __int64)1 << (itm->_iSpell - 1);
+				spl |= SPELLBIT(itm->_iSpell);
 			}
 
 			if (itm->_iMagical == ITEM_QUALITY_NORMAL || itm->_iIdentified) {
@@ -906,7 +906,7 @@ void CalcPlrItemVals(int p, BOOL Loadgfx)
 
 	// check if the current RSplType is a valid/allowed spell
 	if (plr[p]._pRSplType == RSPLTYPE_CHARGES
-	    && !(spl & ((unsigned __int64)1 << (plr[p]._pRSpell - 1)))) {
+	    && !(spl & SPELLBIT(plr[p]._pRSpell))) {
 		plr[p]._pRSpell = SPL_INVALID;
 		plr[p]._pRSplType = RSPLTYPE_INVALID;
 		force_redraw = 255;
@@ -1154,18 +1154,18 @@ void CalcPlrScrolls(int p)
 	for (i = 0; i < plr[p]._pNumInv; i++) {
 		if (plr[p].InvList[i]._itype != ITYPE_NONE && (plr[p].InvList[i]._iMiscId == IMISC_SCROLL || plr[p].InvList[i]._iMiscId == IMISC_SCROLLT)) {
 			if (plr[p].InvList[i]._iStatFlag)
-				plr[p]._pScrlSpells |= (__int64)1 << (plr[p].InvList[i]._iSpell - 1);
+				plr[p]._pScrlSpells |= SPELLBIT(plr[p].InvList[i]._iSpell);
 		}
 	}
 
 	for (j = 0; j < MAXBELTITEMS; j++) {
 		if (plr[p].SpdList[j]._itype != ITYPE_NONE && (plr[p].SpdList[j]._iMiscId == IMISC_SCROLL || plr[p].SpdList[j]._iMiscId == IMISC_SCROLLT)) {
 			if (plr[p].SpdList[j]._iStatFlag)
-				plr[p]._pScrlSpells |= (__int64)1 << (plr[p].SpdList[j]._iSpell - 1);
+				plr[p]._pScrlSpells |= SPELLBIT(plr[p].SpdList[j]._iSpell);
 		}
 	}
 	if (plr[p]._pRSplType == RSPLTYPE_SCROLL) {
-		if (!(plr[p]._pScrlSpells & 1 << (plr[p]._pRSpell - 1))) {
+		if (!(plr[p]._pScrlSpells & 1 << (plr[p]._pRSpell - 1))) { // BUGFIX: apply SPELLBIT macro
 			plr[p]._pRSpell = SPL_INVALID;
 			plr[p]._pRSplType = RSPLTYPE_INVALID;
 			force_redraw = 255;
@@ -1179,7 +1179,7 @@ void CalcPlrStaff(int p)
 	if (plr[p].InvBody[INVLOC_HAND_LEFT]._itype != ITYPE_NONE
 	    && plr[p].InvBody[INVLOC_HAND_LEFT]._iStatFlag
 	    && plr[p].InvBody[INVLOC_HAND_LEFT]._iCharges > 0) {
-		plr[p]._pISpells |= (__int64)1 << (plr[p].InvBody[INVLOC_HAND_LEFT]._iSpell - 1);
+		plr[p]._pISpells |= SPELLBIT(plr[p].InvBody[INVLOC_HAND_LEFT]._iSpell);
 	}
 }
 
@@ -4681,8 +4681,8 @@ void UseItem(int p, int Mid, int spl)
 		}
 		break;
 	case IMISC_BOOK:
-		plr[p]._pMemSpells |= (__int64)1 << (spl - 1);
-		if (plr[p]._pSplLvl[spl] < 15)
+		plr[p]._pMemSpells |= SPELLBIT(spl);
+		if (plr[p]._pSplLvl[spl] < MAX_SPELL_LEVEL)
 			plr[p]._pSplLvl[spl]++;
 		plr[p]._pMana += spelldata[spl].sManaCost << 6;
 		if (plr[p]._pMana > plr[p]._pMaxMana)
diff --git a/Source/objects.cpp b/Source/objects.cpp
index 023f5651a..edab7e061 100644
--- a/Source/objects.cpp
+++ b/Source/objects.cpp
@@ -3064,8 +3064,8 @@ void OperateBook(int pnum, int i)
 		return;
 
 	if (setlvlnum == SL_BONECHAMB) {
-		plr[myplr]._pMemSpells |= ((__int64)1 << (SPL_GUARDIAN - 1));
-		if (plr[pnum]._pSplLvl[SPL_GUARDIAN] < 15)
+		plr[myplr]._pMemSpells |= SPELLBIT(SPL_GUARDIAN);
+		if (plr[pnum]._pSplLvl[SPL_GUARDIAN] < MAX_SPELL_LEVEL)
 			plr[myplr]._pSplLvl[SPL_GUARDIAN]++;
 		quests[Q_SCHAMB]._qactive = QUEST_DONE;
 		if (!deltaload)
@@ -3743,22 +3743,22 @@ void OperateShrine(int pnum, int i, int sType)
 		spell = 1;
 		spells = plr[pnum]._pMemSpells;
 		for (j = 0; j < MAX_SPELLS; j++) {
-			if (spells & spell)
+			if (spell & spells)
 				cnt++;
 			spell <<= 1;
 		}
 		if (cnt > 1) {
 			spell = 1;
-			for (j = 1; j <= MAX_SPELLS; j++) {
+			for (j = SPL_FIREBOLT; j <= MAX_SPELLS; j++) { // BUGFIX: < MAX_SPELLS, there is no spell with MAX_SPELLS index
 				if (plr[pnum]._pMemSpells & spell) {
-					if (plr[pnum]._pSplLvl[j] < 15)
+					if (plr[pnum]._pSplLvl[j] < MAX_SPELL_LEVEL)
 						plr[pnum]._pSplLvl[j]++;
 				}
 				spell <<= 1;
 			}
 			do {
 				r = random_(0, MAX_SPELLS);
-			} while (!(plr[pnum]._pMemSpells & ((__int64)1 << r)));
+			} while (!(plr[pnum]._pMemSpells & SPELLBIT(r + 1)));
 			if (plr[pnum]._pSplLvl[r] >= 2)
 				plr[pnum]._pSplLvl[r] -= 2;
 			else
@@ -3789,10 +3789,10 @@ void OperateShrine(int pnum, int i, int sType)
 			return;
 		if (pnum != myplr)
 			return;
-		plr[pnum]._pMemSpells |= (__int64)1 << (SPL_FIREBOLT - 1);
-		if (plr[pnum]._pSplLvl[SPL_FIREBOLT] < 15)
+		plr[pnum]._pMemSpells |= SPELLBIT(SPL_FIREBOLT);
+		if (plr[pnum]._pSplLvl[SPL_FIREBOLT] < MAX_SPELL_LEVEL)
 			plr[pnum]._pSplLvl[SPL_FIREBOLT]++;
-		if (plr[pnum]._pSplLvl[SPL_FIREBOLT] < 15)
+		if (plr[pnum]._pSplLvl[SPL_FIREBOLT] < MAX_SPELL_LEVEL)
 			plr[pnum]._pSplLvl[SPL_FIREBOLT]++;
 		t = plr[pnum]._pMaxManaBase / 10;
 		v1 = plr[pnum]._pMana - plr[pnum]._pManaBase;
@@ -3922,10 +3922,10 @@ void OperateShrine(int pnum, int i, int sType)
 	case SHRINE_SACRED:
 		if (deltaload || pnum != myplr)
 			return;
-		plr[pnum]._pMemSpells |= (__int64)1 << (SPL_CBOLT - 1);
-		if (plr[pnum]._pSplLvl[SPL_CBOLT] < 15)
+		plr[pnum]._pMemSpells |= SPELLBIT(SPL_CBOLT);
+		if (plr[pnum]._pSplLvl[SPL_CBOLT] < MAX_SPELL_LEVEL)
 			plr[pnum]._pSplLvl[SPL_CBOLT]++;
-		if (plr[pnum]._pSplLvl[SPL_CBOLT] < 15)
+		if (plr[pnum]._pSplLvl[SPL_CBOLT] < MAX_SPELL_LEVEL)
 			plr[pnum]._pSplLvl[SPL_CBOLT]++;
 		t = plr[pnum]._pMaxManaBase / 10;
 		v1 = plr[pnum]._pMana - plr[pnum]._pManaBase;
@@ -4025,10 +4025,10 @@ void OperateShrine(int pnum, int i, int sType)
 			return;
 		if (pnum != myplr)
 			return;
-		plr[pnum]._pMemSpells |= (__int64)1 << (SPL_HBOLT - 1);
-		if (plr[pnum]._pSplLvl[SPL_HBOLT] < 15)
+		plr[pnum]._pMemSpells |= SPELLBIT(SPL_HBOLT);
+		if (plr[pnum]._pSplLvl[SPL_HBOLT] < MAX_SPELL_LEVEL)
 			plr[pnum]._pSplLvl[SPL_HBOLT]++;
-		if (plr[pnum]._pSplLvl[SPL_HBOLT] < 15)
+		if (plr[pnum]._pSplLvl[SPL_HBOLT] < MAX_SPELL_LEVEL)
 			plr[pnum]._pSplLvl[SPL_HBOLT]++;
 		t = plr[pnum]._pMaxManaBase / 10;
 		v1 = plr[pnum]._pMana - plr[pnum]._pManaBase;
diff --git a/Source/player.cpp b/Source/player.cpp
index 23a285e7a..3dbd23826 100644
--- a/Source/player.cpp
+++ b/Source/player.cpp
@@ -819,17 +819,17 @@ void CreatePlayer(int pnum, char c)
 	plr[pnum]._pInfraFlag = FALSE;
 
 	if (c == PC_WARRIOR) {
-		plr[pnum]._pAblSpells = (__int64)1 << (SPL_REPAIR - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_REPAIR);
 #ifndef SPAWN
 	} else if (c == PC_ROGUE) {
-		plr[pnum]._pAblSpells = (__int64)1 << (SPL_DISARM - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_DISARM);
 	} else if (c == PC_SORCERER) {
-		plr[pnum]._pAblSpells = (__int64)1 << (SPL_RECHARGE - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_RECHARGE);
 #endif
 	}
 
 	if (c == PC_SORCERER) {
-		plr[pnum]._pMemSpells = (__int64)1 << (SPL_FIREBOLT - 1);
+		plr[pnum]._pMemSpells = SPELLBIT(SPL_FIREBOLT);
 	} else {
 		plr[pnum]._pMemSpells = 0;
 	}
@@ -1123,19 +1123,19 @@ void InitPlayer(int pnum, BOOL FirstTime)
 	}
 
 	if (plr[pnum]._pClass == PC_WARRIOR) {
-		plr[pnum]._pAblSpells = 1 << (SPL_REPAIR - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_REPAIR);
 #ifndef SPAWN
 	} else if (plr[pnum]._pClass == PC_ROGUE) {
-		plr[pnum]._pAblSpells = 1 << (SPL_DISARM - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_DISARM);
 	} else if (plr[pnum]._pClass == PC_SORCERER) {
-		plr[pnum]._pAblSpells = 1 << (SPL_RECHARGE - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_RECHARGE);
 #ifdef HELLFIRE
 	} else if (plr[pnum]._pClass == PC_MONK) {
-		plr[pnum]._pAblSpells = (unsigned __int64)1 << (SPL_SEARCH - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_SEARCH);
 	} else if (plr[pnum]._pClass == PC_BARD) {
-		plr[pnum]._pAblSpells = 1 << (SPL_IDENTIFY - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_IDENTIFY);
 	} else if (plr[pnum]._pClass == PC_BARBARIAN) {
-		plr[pnum]._pAblSpells = 1 << (SPL_BLODBOIL - 1);
+		plr[pnum]._pAblSpells = SPELLBIT(SPL_BLODBOIL);
 #endif
 #endif
 	}
@@ -3269,7 +3269,7 @@ BOOL PM_DoSpell(int pnum)
 		if (!plr[pnum]._pSplFrom) {
 			if (plr[pnum]._pRSplType == RSPLTYPE_SCROLL) {
 				if (!(plr[pnum]._pScrlSpells
-				        & (unsigned __int64)1 << (plr[pnum]._pRSpell - 1))) {
+				        & SPELLBIT(plr[pnum]._pRSpell))) {
 					plr[pnum]._pRSpell = SPL_INVALID;
 					plr[pnum]._pRSplType = RSPLTYPE_INVALID;
 					force_redraw = 255;
@@ -3278,7 +3278,7 @@ BOOL PM_DoSpell(int pnum)
 
 			if (plr[pnum]._pRSplType == RSPLTYPE_CHARGES) {
 				if (!(plr[pnum]._pISpells
-				        & (unsigned __int64)1 << (plr[pnum]._pRSpell - 1))) {
+				        & SPELLBIT(plr[pnum]._pRSpell))) {
 					plr[pnum]._pRSpell = SPL_INVALID;
 					plr[pnum]._pRSplType = RSPLTYPE_INVALID;
 					force_redraw = 255;
@@ -3780,9 +3780,9 @@ void ValidatePlayer()
 
 	for (b = 1; b < MAX_SPELLS; b++) {
 		if (spelldata[b].sBookLvl != -1) {
-			msk |= (__int64)1 << (b - 1);
-			if (plr[myplr]._pSplLvl[b] > 15)
-				plr[myplr]._pSplLvl[b] = 15;
+			msk |= SPELLBIT(b);
+			if (plr[myplr]._pSplLvl[b] > MAX_SPELL_LEVEL)
+				plr[myplr]._pSplLvl[b] = MAX_SPELL_LEVEL;
 		}
 	}
 
diff --git a/defs.h b/defs.h
index c81519c23..82982badf 100644
--- a/defs.h
+++ b/defs.h
@@ -43,6 +43,8 @@
 #define MAX_LVLMTYPES			16
 #define MAX_SPELLS				37
 #endif
+#define MAX_SPELL_LEVEL			15
+#define SPELLBIT(s) ((__int64)1 << (s - 1))
 
 #define MAX_CHUNKS				(MAX_LVLS + 5)