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src/basic/peek_line_pointer_null_check_46K_50K.s

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+// #LAYOUT# STD *       #TAKE-HIGH
+// #LAYOUT# *   BASIC_0 #TAKE-HIGH
+// #LAYOUT# *   *       #IGNORE
+
+// This has to go $E000 or above - routine below banks out the main BASIC ROM!
+
+// Return C=1 if a pointer in BASIC memory space is NULL, else C=0
+// X = ZP pointer to check
+
+
+#if CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+
+peek_line_pointer_null_check:
+
+	// Unmap BASIC lower ROM
+
+	lda #$26
+	sta CPU_R6510
+
+	// Check the pointer
+
+	ldy #$01                 // for non-NULL pointer, high byte is almost certainly not NULL
+	lda (OLDTXT),y
+	bne !+
+	
+	dey
+	lda (OLDTXT),y
+	bne !+
+
+	// Pointer is NULL
+
+	lda #$27
+	sta CPU_R6510
+
+	clc
+	rts
+!:
+	// Pointer not NULL
+
+	lda #$27
+	sta CPU_R6510
+
+	sec
+	rts
+
+#endif