diff --git a/build.sh b/build.sh
index 4469b0a..a73d3d4 100755
--- a/build.sh
+++ b/build.sh
@@ -1,4 +1,3 @@
 #!/bin/sh
 
 xa -W -C -v -O ASCII -S -c src/apple1serial.xa -l apple1serial.label -o apple1serial.bin
-#xa -W -C -v -O ASCII -S -c src/serialmonitor.xa -l serialmonitor.label -o serialmonitor.bin
diff --git a/firmware/main.c b/firmware/main.c
index 312b1a9..7117533 100644
--- a/firmware/main.c
+++ b/firmware/main.c
@@ -21,21 +21,21 @@
 #define DATA6 PD3
 #define DATA7 PD4
 
-#define BUFFER_SIZE 8
+#define BUFFER_SIZE 100
 
 typedef enum {NONE, READ, WRITE} mode_t;
-mode_t mode = NONE;
+volatile mode_t mode = NONE;
 
-uint8_t data = 0x00;
-uint8_t upper_part = 0x00;
-uint8_t lower_part = 0x00;
+volatile uint8_t data = 0x00;
+volatile uint8_t upper_part = 0x00;
+volatile uint8_t lower_part = 0x00;
 
-uint8_t data_write_interrupt_count = 0;
-uint8_t data_read_interrupt_count = 0;
+volatile uint8_t data_write_interrupt_count = 0;
+volatile uint8_t data_read_interrupt_count = 0;
 
 // receive buffer
-uint8_t receive_buffer[BUFFER_SIZE];
-int8_t receive_index = -1;
+volatile uint8_t receive_buffer[BUFFER_SIZE];
+volatile int8_t receive_index = -1;
 
 // Registers: PCMSK0, PCMSK1, PCMSK2 :registers that enable or disable pin-change interrupts
 // on individual pins
diff --git a/src/apple1serial.xa b/src/apple1serial.xa
index 56fdeb5..ac77107 100644
--- a/src/apple1serial.xa
+++ b/src/apple1serial.xa
@@ -1,4 +1,4 @@
- ;End address of dump block
+;End address of dump block
 #define hex1_l $24
 #define hex1_h $25
 
@@ -147,7 +147,10 @@ read
     beq read_ready
     lda serial_read     ;Enable read mode, this can be done quick, without reset
 read_ready
+    txa                 ;Preserve X on stack
+    pha
     ldy #$00
+    ldx #hex2_l
 
 read_byte
     lda serial_ready
@@ -165,15 +168,14 @@ read_byte
 
     lda #last_command_read ;Set last command to READ
     sta last_command
+
+    pla                 ;Restore X from stack
+    tax
+
     jmp next_cmd        ;Read is completed, proceed to next command
 
 read_next
-    txa                 ;Preserve X on stack
-    pha
-    ldx #hex2_l
     jsr increment_16bit ;Increment destination address
-    pla                 ;Restore X from stack
-    tax
     jmp read_byte
 
 ;-------------------------------------------------------------------------
@@ -189,6 +191,9 @@ write
                         ;and prevents from polluting the output while setting the write mode
     sta serial_write    ;Enable write mode
 write_ready
+    txa                 ;Preserve X on stack
+    pha
+    ldx #hex2_l
     ldy #$00
 
 write_byte
@@ -207,15 +212,14 @@ write_byte
 
     lda #last_command_write ;Set last command to WRITE
     sta last_command
+
+    pla                 ;Restore X from stack
+    tax
+
     jmp next_cmd        ;Write is completed, proceed to next command
 
 write_next
-    txa                 ;Preserve X on stack
-    pha
-    ldx #hex2_l
     jsr increment_16bit ;Increment destination address
-    pla                 ;Restore X from stack
-    tax
     jmp write_byte
 
 ;-------------------------------------------------------------------------
@@ -247,11 +251,13 @@ reset_loop              ; ~((2 + 2 + 3) * 255 + 2 + 3) * 255 =
     rts
 
 increment_16bit
+    txa
     inc $00,X
     bne increment_16bit_done
     inx
     inc $00,X
 increment_16bit_done
+    tax
     rts
 
 ;-------------------------------------------------------------------------