diff --git a/windows.c b/windows.c
new file mode 100644
index 0000000..762054a
--- /dev/null
+++ b/windows.c
@@ -0,0 +1,673 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2018 IBM Corp.
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <limits.h>
+#include <poll.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syslog.h>
+#include <signal.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/timerfd.h>
+#include <sys/types.h>
+#include <time.h>
+#include <unistd.h>
+#include <inttypes.h>
+#include <mtd/mtd-abi.h>
+
+#include "mbox.h"
+#include "common.h"
+#include "mboxd_msg.h"
+#include "windows.h"
+#include "flash.h"
+
+/* Initialisation Functions */
+
+/*
+ * init_window_state() - Initialise a new window to a known state
+ * @window:	The window to initialise
+ * @size:	The size of the window
+ */
+static void init_window_state(struct window_context *window, uint32_t size)
+{
+	window->mem = NULL;
+	window->flash_offset = FLASH_OFFSET_UNINIT;
+	window->size = size;
+	window->dirty_bmap = NULL;
+	window->age = 0;
+}
+
+/*
+ * init_window_mem() - Divide the reserved memory region among the windows
+ * @context:	The mbox context pointer
+ *
+ * Return:	0 on success otherwise negative error code
+ */
+static int init_window_mem(struct mbox_context *context)
+{
+	void *mem_location = context->mem;
+	int i;
+
+	/*
+	 * Carve up the reserved memory region and allocate it to each of the
+	 * windows. The windows are placed one after the other in ascending
+	 * order, so the first window will be first in memory and so on. We
+	 * shouldn't have allocated more windows than we have memory, but if we
+	 * did we will error out here
+	 */
+	for (i = 0; i < context->windows.num; i++) {
+		uint32_t size = context->windows.window[i].size;
+		MSG_DBG("Window %d @ %p for size 0x%.8x\n", i,
+			mem_location, size);
+		context->windows.window[i].mem = mem_location;
+		mem_location += size;
+		if (mem_location > (context->mem + context->mem_size)) {
+			/* Tried to allocate window past the end of memory */
+			MSG_ERR("Total size of windows exceeds reserved mem\n");
+			MSG_ERR("Try smaller or fewer windows\n");
+			MSG_ERR("Mem size: 0x%.8x\n", context->mem_size);
+			return -1;
+		}
+	}
+
+	return 0;
+}
+/*
+ * init_windows() - Initalise the window cache
+ * @context:    The mbox context pointer
+ *
+ * Return:      0 on success otherwise negative
+ */
+int init_windows(struct mbox_context *context)
+{
+	int i;
+
+	/* Check if window size and number set - otherwise set to default */
+	if (!context->windows.default_size) {
+		/* Default to 1MB windows */
+		context->windows.default_size = 1 << 20;
+	}
+	MSG_INFO("Window size: 0x%.8x\n", context->windows.default_size);
+	if (!context->windows.num) {
+		/* Use the entire reserved memory region by default */
+		context->windows.num = context->mem_size /
+				       context->windows.default_size;
+	}
+	MSG_INFO("Number of windows: %d\n", context->windows.num);
+
+	context->windows.window = calloc(context->windows.num,
+					 sizeof(*context->windows.window));
+	if (!context->windows.window) {
+		MSG_ERR("Memory allocation failed\n");
+		return -1;
+	}
+
+	for (i = 0; i < context->windows.num; i++) {
+		init_window_state(&context->windows.window[i],
+				  context->windows.default_size);
+	}
+
+	return init_window_mem(context);
+}
+
+/*
+ * free_windows() - Free the window cache
+ * @context:	The mbox context pointer
+ */
+void free_windows(struct mbox_context *context)
+{
+	int i;
+
+	/* Check window cache has actually been allocated */
+	if (context->windows.window) {
+		for (i = 0; i < context->windows.num; i++) {
+			free(context->windows.window[i].dirty_bmap);
+		}
+		free(context->windows.window);
+	}
+}
+
+/* Write from Window Functions */
+
+/*
+ * write_from_window_v1() - Handle writing when erase and block size differ
+ * @context:		The mbox context pointer
+ * @offset_bytes:	The offset in the current window to write from (bytes)
+ * @count_bytes:	Number of bytes to write
+ *
+ * Handle a write_from_window for dirty memory when block_size is less than the
+ * flash erase size
+ * This requires us to be a bit careful because we might have to erase more
+ * than we want to write which could result in data loss if we don't have the
+ * entire portion of flash to be erased already saved in memory (for us to
+ * write back after the erase)
+ *
+ * Return:	0 on success otherwise negative error code
+ */
+int write_from_window_v1(struct mbox_context *context,
+			 uint32_t offset_bytes, uint32_t count_bytes)
+{
+	int rc;
+	uint32_t flash_offset;
+	struct window_context low_mem = { 0 }, high_mem = { 0 };
+
+	/* Find where in phys flash this is based on the window.flash_offset */
+	flash_offset = context->current->flash_offset + offset_bytes;
+
+	/*
+	 * low_mem.flash_offset = erase boundary below where we're writing
+	 * low_mem.size = size from low_mem.flash_offset to where we're writing
+	 *
+	 * high_mem.flash_offset = end of where we're writing
+	 * high_mem.size = size from end of where we're writing to next erase
+	 * 		   boundary
+	 */
+	low_mem.flash_offset = align_down(flash_offset,
+					  context->mtd_info.erasesize);
+	low_mem.size = flash_offset - low_mem.flash_offset;
+	high_mem.flash_offset = flash_offset + count_bytes;
+	high_mem.size = align_up(high_mem.flash_offset,
+				 context->mtd_info.erasesize) -
+			high_mem.flash_offset;
+
+	/*
+	 * Check if we already have a copy of the required flash areas in
+	 * memory as part of the existing window
+	 */
+	if (low_mem.flash_offset < context->current->flash_offset) {
+		/* Before the start of our current window */
+		low_mem.mem = malloc(low_mem.size);
+		if (!low_mem.mem) {
+			MSG_ERR("Unable to allocate memory\n");
+			return -MBOX_R_SYSTEM_ERROR;
+		}
+		rc = flash_copy(context, low_mem.flash_offset,
+				low_mem.mem, low_mem.size);
+		if (rc < 0) {
+			goto out;
+		}
+	}
+	if ((high_mem.flash_offset + high_mem.size) >
+	    (context->current->flash_offset + context->current->size)) {
+		/* After the end of our current window */
+		high_mem.mem = malloc(high_mem.size);
+		if (!high_mem.mem) {
+			MSG_ERR("Unable to allocate memory\n");
+			rc = -MBOX_R_SYSTEM_ERROR;
+			goto out;
+		}
+		rc = flash_copy(context, high_mem.flash_offset,
+				high_mem.mem, high_mem.size);
+		if (rc < 0) {
+			goto out;
+		}
+	}
+
+	/*
+	 * We need to erase the flash from low_mem.flash_offset->
+	 * high_mem.flash_offset + high_mem.size
+	 */
+	rc = flash_erase(context, low_mem.flash_offset,
+			 (high_mem.flash_offset - low_mem.flash_offset) +
+			 high_mem.size);
+	if (rc < 0) {
+		MSG_ERR("Couldn't erase flash\n");
+		goto out;
+	}
+
+	/* Write back over the erased area */
+	if (low_mem.mem) {
+		/* Exceed window at the start */
+		rc = flash_write(context, low_mem.flash_offset, low_mem.mem,
+				 low_mem.size);
+		if (rc < 0) {
+			goto out;
+		}
+	}
+	rc = flash_write(context, flash_offset,
+			 context->current->mem + offset_bytes, count_bytes);
+	if (rc < 0) {
+		goto out;
+	}
+	/*
+	 * We still need to write the last little bit that we erased - it's
+	 * either in the current window or the high_mem window.
+	 */
+	if (high_mem.mem) {
+		/* Exceed window at the end */
+		rc = flash_write(context, high_mem.flash_offset, high_mem.mem,
+				 high_mem.size);
+		if (rc < 0) {
+			goto out;
+		}
+	} else {
+		/* Write from the current window - it's atleast that big */
+		rc = flash_write(context, high_mem.flash_offset,
+				 context->current->mem + offset_bytes +
+				 count_bytes, high_mem.size);
+		if (rc < 0) {
+			goto out;
+		}
+	}
+
+out:
+	free(low_mem.mem);
+	free(high_mem.mem);
+	return rc;
+}
+
+/*
+ * write_from_window() - Write back to the flash from the current window
+ * @context:		The mbox context pointer
+ * @offset_bytes:	The offset in the current window to write from (blocks)
+ * @count_bytes:	Number of blocks to write
+ * @type:		Whether this is an erase & write or just an erase
+ *
+ * Return:	0 on success otherwise negative error code
+ */
+int write_from_window(struct mbox_context *context, uint32_t offset,
+		      uint32_t count, uint8_t type)
+{
+	int rc;
+	uint32_t flash_offset, count_bytes = count << context->block_size_shift;
+	uint32_t offset_bytes = offset << context->block_size_shift;
+
+	switch (type) {
+	case WINDOW_ERASED: /* >= V2 ONLY -> block_size == erasesize */
+		flash_offset = context->current->flash_offset + offset_bytes;
+		rc = flash_erase(context, flash_offset, count_bytes);
+		if (rc < 0) {
+			MSG_ERR("Couldn't erase flash\n");
+			return rc;
+		}
+		break;
+	case WINDOW_DIRTY:
+		/*
+		 * For protocol V1, block_size may be smaller than erase size
+		 * so we have a special function to make sure that we do this
+		 * correctly without losing data.
+		 */
+		if (log_2(context->mtd_info.erasesize) !=
+						context->block_size_shift) {
+			return write_from_window_v1(context, offset_bytes,
+						    count_bytes);
+		}
+		flash_offset = context->current->flash_offset + offset_bytes;
+
+		/* Erase the flash */
+		rc = flash_erase(context, flash_offset, count_bytes);
+		if (rc < 0) {
+			return rc;
+		}
+
+		/* Write to the erased flash */
+		rc = flash_write(context, flash_offset,
+				 context->current->mem + offset_bytes,
+				 count_bytes);
+		if (rc < 0) {
+			return rc;
+		}
+
+		break;
+	default:
+		/* We shouldn't be able to get here */
+		MSG_ERR("Write from window with invalid type: %d\n", type);
+		return -MBOX_R_SYSTEM_ERROR;
+	}
+
+	return 0;
+}
+
+/* Window Management Functions */
+
+/*
+ * alloc_window_dirty_bytemap() - (re)allocate all the window dirty bytemaps
+ * @context:		The mbox context pointer
+ */
+void alloc_window_dirty_bytemap(struct mbox_context *context)
+{
+	struct window_context *cur;
+	int i;
+
+	for (i = 0; i < context->windows.num; i++) {
+		cur = &context->windows.window[i];
+		/* There may already be one allocated */
+		free(cur->dirty_bmap);
+		/* Allocate the new one */
+		cur->dirty_bmap = calloc((cur->size >>
+					  context->block_size_shift),
+					 sizeof(*cur->dirty_bmap));
+	}
+}
+
+/*
+ * set_window_bytemap() - Set the window bytemap
+ * @context:	The mbox context pointer
+ * @cur:	The window to set the bytemap of
+ * @offset:	Where in the window to set the bytemap (blocks)
+ * @size:	The number of blocks to set
+ * @val:	The value to set the bytemap to
+ *
+ * Return:	0 on success otherwise negative error code
+ */
+int set_window_bytemap(struct mbox_context *context, struct window_context *cur,
+		       uint32_t offset, uint32_t size, uint8_t val)
+{
+	if (offset + size > (cur->size >> context->block_size_shift)) {
+		MSG_ERR("Tried to set window bytemap past end of window\n");
+		MSG_ERR("Requested offset: 0x%x size: 0x%x window size: 0x%x\n",
+			offset << context->block_size_shift,
+			size << context->block_size_shift,
+			cur->size << context->block_size_shift);
+		return -EACCES;
+	}
+
+	memset(cur->dirty_bmap + offset, val, size);
+	return 0;
+}
+
+/*
+ * close_current_window() - Close the current (active) window
+ * @context:   		The mbox context pointer
+ * @set_bmc_event:	Whether to set the bmc event bit
+ * @flags:		Flags as defined for a close command in the protocol
+ *
+ * This closes the current window. If the host has requested the current window
+ * be closed then we don't need to set the bmc event bit
+ * (set_bmc_event == false), otherwise if the current window has been closed
+ * without the host requesting it the bmc event bit must be set to indicate this
+ * to the host (set_bmc_event == true).
+ */
+void close_current_window(struct mbox_context *context, bool set_bmc_event,
+			  uint8_t flags)
+{
+	MSG_DBG("Close current window, flags: 0x%.2x\n", flags);
+
+	if (set_bmc_event) {
+		set_bmc_events(context, BMC_EVENT_WINDOW_RESET, SET_BMC_EVENT);
+	}
+
+	if (flags & FLAGS_SHORT_LIFETIME) {
+		context->current->age = 0;
+	}
+
+	context->current = NULL;
+	context->current_is_write = false;
+}
+
+/*
+ * reset_window() - Reset a window context to a well defined default state
+ * @context:   	The mbox context pointer
+ * @window:	The window to reset
+ */
+void reset_window(struct mbox_context *context, struct window_context *window)
+{
+	window->flash_offset = FLASH_OFFSET_UNINIT;
+	window->size = context->windows.default_size;
+	if (window->dirty_bmap) { /* Might not have been allocated */
+		set_window_bytemap(context, window, 0,
+				   window->size >> context->block_size_shift,
+				   WINDOW_CLEAN);
+	}
+	window->age = 0;
+}
+
+/*
+ * reset_all_windows() - Reset all windows to a well defined default state
+ * @context:		The mbox context pointer
+ * @set_bmc_event:	If any state change should be indicated to the host
+ */
+void reset_all_windows(struct mbox_context *context, bool set_bmc_event)
+{
+	int i;
+
+	MSG_DBG("Resetting all windows\n");
+	/* We might have an open window which needs closing */
+	if (context->current) {
+		close_current_window(context, set_bmc_event, FLAGS_NONE);
+	}
+	for (i = 0; i < context->windows.num; i++) {
+		reset_window(context, &context->windows.window[i]);
+	}
+
+	context->windows.max_age = 0;
+}
+
+/*
+ * find_oldest_window() - Find the oldest (Least Recently Used) window
+ * @context:		The mbox context pointer
+ *
+ * Return:	Pointer to the least recently used window
+ */
+struct window_context *find_oldest_window(struct mbox_context *context)
+{
+	struct window_context *oldest = NULL, *cur;
+	uint32_t min_age = context->windows.max_age + 1;
+	int i;
+
+	for (i = 0; i < context->windows.num; i++) {
+		cur = &context->windows.window[i];
+
+		if (cur->age < min_age) {
+			min_age = cur->age;
+			oldest = cur;
+		}
+	}
+
+	return oldest;
+}
+
+/*
+ * find_largest_window() - Find the largest window in the window cache
+ * @context:	The mbox context pointer
+ *
+ * Return:	The largest window
+ */
+struct window_context *find_largest_window(struct mbox_context *context)
+{
+	struct window_context *largest = NULL, *cur;
+	uint32_t max_size = 0;
+	int i;
+
+	for (i = 0; i < context->windows.num; i++) {
+		cur = &context->windows.window[i];
+
+		if (cur->size > max_size) {
+			max_size = cur->size;
+			largest = cur;
+		}
+	}
+
+	return largest;
+}
+
+/*
+ * search_windows() - Search the window cache for a window containing offset
+ * @context:	The mbox context pointer
+ * @offset:	Absolute flash offset to search for (bytes)
+ * @exact:	If the window must exactly map the requested offset
+ *
+ * This will search the cache of windows for one containing the requested
+ * offset. For V1 of the protocol windows must exactly map the offset since we
+ * can't tell the host how much of its request we actually mapped and it will
+ * thus assume it can access window->size from the offset we give it.
+ *
+ * Return:	Pointer to a window containing the requested offset otherwise
+ *		NULL
+ */
+struct window_context *search_windows(struct mbox_context *context,
+				      uint32_t offset, bool exact)
+{
+	struct window_context *cur;
+	int i;
+
+	MSG_DBG("Searching for window which contains 0x%.8x %s\n",
+		offset, exact ? "exactly" : "");
+	for (i = 0; i < context->windows.num; i++) {
+		cur = &context->windows.window[i];
+		if (cur->flash_offset == FLASH_OFFSET_UNINIT) {
+			/* Uninitialised Window */
+			if (offset == FLASH_OFFSET_UNINIT) {
+				return cur;
+			}
+			continue;
+		}
+		if ((offset >= cur->flash_offset) &&
+		    (offset < (cur->flash_offset + cur->size))) {
+			if (exact && (cur->flash_offset != offset)) {
+				continue;
+			}
+			/* This window contains the requested offset */
+			cur->age = ++(context->windows.max_age);
+			return cur;
+		}
+	}
+
+	return NULL;
+}
+
+/*
+ * create_map_window() - Create a window mapping which maps the requested offset
+ * @context:		The mbox context pointer
+ * @this_window:	A pointer to update to the "new" window
+ * @offset:		Absolute flash offset to create a mapping for (bytes)
+ * @exact:		If the window must exactly map the requested offset
+ *
+ * This is used to create a window mapping for the requested offset when there
+ * is no existing window in the cache which satisfies the offset. This involves
+ * choosing an existing window from the window cache to evict so we can use it
+ * to store the flash contents from the requested offset, we then point the
+ * caller to that window since it now maps their request.
+ *
+ * Return:	0 on success otherwise negative error code
+ */
+int create_map_window(struct mbox_context *context,
+		      struct window_context **this_window, uint32_t offset,
+		      bool exact)
+{
+	struct window_context *cur = NULL;
+	int rc;
+
+	MSG_DBG("Creating window which maps 0x%.8x %s\n", offset,
+		exact ? "exactly" : "");
+
+	/* Search for an uninitialised window, use this before evicting */
+	cur = search_windows(context, FLASH_OFFSET_UNINIT, true);
+
+	/* No uninitialised window found, we need to choose one to "evict" */
+	if (!cur) {
+		MSG_DBG("No uninitialised window, evicting one\n");
+		cur = find_oldest_window(context);
+		reset_window(context, cur);
+	}
+
+/*
+ * In case of the virtual pnor, as of now it's possible that a window may
+ * have content less than it's max size. We basically copy one flash partition
+ * per window, and some partitions are smaller than the max size. An offset
+ * right after such a small partition ends should lead to new mapping. The code
+ * below prevents that.
+ */
+#ifndef VIRTUAL_PNOR_ENABLED
+	if (!exact) {
+		/*
+		 * It would be nice to align the offsets which we map to window
+		 * size, this will help prevent overlap which would be an
+		 * inefficient use of our reserved memory area (we would like
+		 * to "cache" as much of the acutal flash as possible in
+		 * memory). If we're protocol V1 however we must ensure the
+		 * offset requested is exactly mapped.
+		 */
+		offset &= ~(cur->size - 1);
+	}
+#endif
+
+	if (offset > context->flash_size) {
+		MSG_ERR("Tried to open read window past flash limit\n");
+		return -MBOX_R_PARAM_ERROR;
+	} else if ((offset + cur->size) > context->flash_size) {
+		/*
+		 * There is V1 skiboot implementations out there which don't
+		 * mask offset with window size, meaning when we have
+		 * window size == flash size we will never allow the host to
+		 * open a window except at 0x0, which isn't always where the
+		 * host requests it. Thus we have to ignore this check and just
+		 * hope the host doesn't access past the end of the window
+		 * (which it shouldn't) for V1 implementations to get around
+		 * this.
+		 */
+		if (context->version == API_VERSION_1) {
+			cur->size = align_down(context->flash_size - offset,
+					       1 << context->block_size_shift);
+		} else {
+			/*
+			 * Allow requests to exceed the flash size, but limit
+			 * the response to the size of the flash.
+			 */
+			cur->size = context->flash_size - offset;
+		}
+	}
+
+	/* Copy from flash into the window buffer */
+	rc = flash_copy(context, offset, cur->mem, cur->size);
+	if (rc < 0) {
+		/* We don't know how much we've copied -> better reset window */
+		reset_window(context, cur);
+		return rc;
+	}
+	/*
+	 * rc isn't guaranteed to be aligned, so align up
+	 *
+	 * FIXME: This should only be the case for the vpnor ToC now, so handle
+	 * it there
+	 */
+	cur->size = align_up(rc, (1ULL << context->block_size_shift));
+	/* Would like a known value, pick 0xFF to it looks like erased flash */
+	memset(cur->mem + rc, 0xFF, cur->size - rc);
+
+	/*
+	 * Since for V1 windows aren't constrained to start at multiples of
+	 * window size it's possible that something already maps this offset.
+	 * Reset any windows which map this offset to avoid coherency problems.
+	 * We just have to check for anything which maps the start or the end
+	 * of the window since all windows are the same size so another window
+	 * cannot map just the middle of this window.
+	 */
+	if (context->version == API_VERSION_1) {
+		uint32_t i;
+
+		MSG_DBG("Checking for window overlap\n");
+
+		for (i = offset; i < (offset + cur->size); i += (cur->size - 1)) {
+			struct window_context *tmp = NULL;
+			do {
+				tmp = search_windows(context, i, false);
+				if (tmp) {
+					reset_window(context, tmp);
+				}
+			} while (tmp);
+		}
+	}
+
+	/* Clear the bytemap of the window just loaded -> we know it's clean */
+	set_window_bytemap(context, cur, 0,
+			   cur->size >> context->block_size_shift,
+			   WINDOW_CLEAN);
+
+	/* Update so we know what's in the window */
+	cur->flash_offset = offset;
+	cur->age = ++(context->windows.max_age);
+	*this_window = cur;
+
+	return 0;
+}