diff --git a/sf-writer/include/ImageAssembler.hpp b/sf-writer/include/ImageAssembler.hpp
index 525b4c4..87f1070 100644
--- a/sf-writer/include/ImageAssembler.hpp
+++ b/sf-writer/include/ImageAssembler.hpp
@@ -15,20 +15,18 @@ class ImageAssembler {
     ImageMetadataBlock* metadata_buffer_;
     std::atomic_int* buffer_status_;
 
-    int read_slot_id_;
-    std::atomic_int write_slot_id_;
-
 public:
     ImageAssembler(const size_t n_modules);
 
     virtual ~ImageAssembler();
 
-    int get_free_slot();
-    void process(const int slot_id,
+    bool is_slot_free(const int slot_id);
+    bool is_slot_full(const int slot_id);
+
+    void process(int slot_id,
                  const int i_module,
                  const BufferBinaryBlock* block_buffer);
 
-    int get_full_slot();
     void free_slot(const int slot_id);
 
     ImageMetadataBlock* get_metadata_buffer(const int slot_id);
diff --git a/sf-writer/src/ImageAssembler.cpp b/sf-writer/src/ImageAssembler.cpp
index b090489..fe271c4 100644
--- a/sf-writer/src/ImageAssembler.cpp
+++ b/sf-writer/src/ImageAssembler.cpp
@@ -15,9 +15,6 @@ ImageAssembler::ImageAssembler(const size_t n_modules) :
     for (size_t i=0; i<IA_N_SLOTS; i++) {
         free_slot(i);
     }
-
-    read_slot_id_ = 0;
-    write_slot_id_ = 0;
 }
 
 ImageAssembler::~ImageAssembler()
@@ -26,20 +23,23 @@ ImageAssembler::~ImageAssembler()
     delete[] metadata_buffer_;
 }
 
-int ImageAssembler::get_free_slot()
+bool ImageAssembler::is_slot_free(const int slot_id)
 {
-    if (buffer_status_[write_slot_id_] > 0) {
-        return write_slot_id_;
-    }
+    return buffer_status_[slot_id % IA_N_SLOTS] > 0;
+}
 
-    return -1;
+bool ImageAssembler::is_slot_full(const int slot_id)
+{
+    return buffer_status_[slot_id % IA_N_SLOTS] == 0;
 }
 
 void ImageAssembler::process(
-        const int slot_id,
+        int slot_id,
         const int i_module,
         const BufferBinaryBlock* block_buffer)
 {
+    slot_id %= IA_N_SLOTS;
+
     // TODO: Temp workaround. Make proper initialization.
     if (i_module == 0) {
         metadata_buffer_[slot_id].block_first_pulse_id =
@@ -72,37 +72,21 @@ void ImageAssembler::process(
         }
     }
 
-    auto previous_status = buffer_status_[slot_id].fetch_sub(1);
-
-    // 1 because fetch is done before subtraction.
-    if (previous_status ==  1) {
-        auto next_write_slot_id = (int)((write_slot_id_+1) % IA_N_SLOTS);
-        write_slot_id_ = next_write_slot_id;
-    }
-}
-
-int ImageAssembler::get_full_slot()
-{
-    if (buffer_status_[read_slot_id_] == 0) {
-        return read_slot_id_;
-    }
-
-    return -1;
+    buffer_status_[slot_id]--;
 }
 
 void ImageAssembler::free_slot(const int slot_id)
 {
-    buffer_status_[slot_id] = n_modules_;
-
-    read_slot_id_ = (int)((read_slot_id_+1) % IA_N_SLOTS);
+    buffer_status_[slot_id % IA_N_SLOTS] = n_modules_;
 }
 
 ImageMetadataBlock* ImageAssembler::get_metadata_buffer(const int slot_id)
 {
-    return &(metadata_buffer_[slot_id]);
+    return &(metadata_buffer_[slot_id % IA_N_SLOTS]);
 }
 
 char* ImageAssembler::get_data_buffer(const int slot_id)
 {
-    return image_buffer_ + (slot_id * image_buffer_slot_n_bytes_);
+    return image_buffer_ +
+           ((slot_id % IA_N_SLOTS) * image_buffer_slot_n_bytes_);
 }
diff --git a/sf-writer/src/main.cpp b/sf-writer/src/main.cpp
index 7978eeb..56bb458 100644
--- a/sf-writer/src/main.cpp
+++ b/sf-writer/src/main.cpp
@@ -26,10 +26,10 @@ void read_buffer(
     BufferBinaryReader block_reader(device, channel_name);
     auto block_buffer = new BufferBinaryBlock();
 
+    int slot_id = 0;
     for (uint64_t block_id:buffer_blocks) {
 
-        int slot_id;
-        while((slot_id = image_assembler.get_free_slot()) == -1) {
+        while(!image_assembler.is_slot_free(slot_id)) {
             this_thread::sleep_for(chrono::milliseconds(
                     WRITER_IMAGE_ASSEMBLER_RETRY_MS));
         }
@@ -45,6 +45,7 @@ void read_buffer(
         start_time = steady_clock::now();
 
         image_assembler.process(slot_id, i_module, block_buffer);
+        slot_id++;
 
         end_time = steady_clock::now();
         uint64_t compose_us_duration = duration_cast<microseconds>(
@@ -116,10 +117,10 @@ int main (int argc, char *argv[])
 
     JFH5Writer writer(output_file, start_pulse_id, stop_pulse_id, n_modules);
 
+    int slot_id = 0;
     for (uint64_t block_id:buffer_blocks) {
 
-        int slot_id;
-        while((slot_id = image_assembler.get_full_slot()) == -1) {
+        while(!image_assembler.is_slot_full(slot_id)) {
             this_thread::sleep_for(chrono::milliseconds(
                     WRITER_IMAGE_ASSEMBLER_RETRY_MS));
         }
@@ -136,6 +137,7 @@ int main (int argc, char *argv[])
                 end_time-start_time).count();
 
         image_assembler.free_slot(slot_id);
+        slot_id++;
 
         cout << "sf_writer:avg_write_us ";
         cout << write_us_duration / BUFFER_BLOCK_SIZE << endl;