diff --git a/libmspack/msoab_decompressor.cs b/libmspack/msoab_decompressor.cs
index a9f2466..933a114 100644
--- a/libmspack/msoab_decompressor.cs
+++ b/libmspack/msoab_decompressor.cs
@@ -1,3 +1,5 @@
+using static SabreTools.Compression.libmspack.oab;
+
 namespace SabreTools.Compression.libmspack
 {
     /// <summary>
@@ -7,7 +9,7 @@ namespace SabreTools.Compression.libmspack
     /// </summary>
     /// <see cref="mspack_create_oab_decompressor()"/> 
     /// <see cref="mspack_destroy_oab_decompressor()"/> 
-    public abstract class msoab_decompressor
+    public unsafe class msoab_decompressor
     {
         public mspack_system system { get; set; }
 
@@ -28,8 +30,150 @@ namespace SabreTools.Compression.libmspack
         /// The filename of the output file. This is passed
         /// directly to mspack_system::open().
         /// </param>
-        /// <returns>An error code, or MSPACK_ERR_OK if successful</returns>
-        public abstract MSPACK_ERR decompress(in string input, in string output);
+        /// <returns>An error code, or MSPACK_ERR.MSPACK_ERR_OK if successful</returns>
+        public MSPACK_ERR decompress(in string input, in string output)
+        {
+            msoab_decompressor_p* self = (msoab_decompressor_p*)_self;
+            mspack_system* sys;
+            mspack_file* infh = NULL;
+            mspack_file* outfh = NULL;
+            byte* buf = NULL;
+            byte[] hdrbuf = new byte[oabhead_SIZEOF];
+            uint block_max, target_size;
+            lzxd_stream* lzx = NULL;
+            mspack_system oabd_sys;
+            oabd_file in_ofh, out_ofh;
+            uint window_bits;
+            int ret = MSPACK_ERR_OK;
+
+            if (!self) return MSPACK_ERR_ARGS;
+            sys = self->system;
+
+            infh = sys->open(sys, input, MSPACK_SYS_OPEN_READ);
+            if (!infh)
+            {
+                ret = MSPACK_ERR_OPEN;
+                goto outlbl;
+            }
+
+            if (sys->read(infh, hdrbuf, oabhead_SIZEOF) != oabhead_SIZEOF)
+            {
+                ret = MSPACK_ERR_READ;
+                goto outlbl;
+            }
+
+            if (EndGetI32(&hdrbuf[oabhead_VersionHi]) != 3 ||
+                EndGetI32(&hdrbuf[oabhead_VersionLo]) != 1)
+            {
+                ret = MSPACK_ERR_SIGNATURE;
+                goto outlbl;
+            }
+
+            block_max = EndGetI32(&hdrbuf[oabhead_BlockMax]);
+            target_size = EndGetI32(&hdrbuf[oabhead_TargetSize]);
+
+            outfh = sys->open(sys, output, MSPACK_SYS_OPEN_WRITE);
+            if (!outfh)
+            {
+                ret = MSPACK_ERR_OPEN;
+                goto outlbl;
+            }
+
+            buf = sys->alloc(sys, self->buf_size);
+            if (!buf)
+            {
+                ret = MSPACK_ERR_NOMEMORY;
+                goto outlbl;
+            }
+
+            oabd_sys = *sys;
+            oabd_sys.read = oabd_sys_read;
+            oabd_sys.write = oabd_sys_write;
+
+            in_ofh.orig_sys = sys;
+            in_ofh.orig_file = infh;
+
+            out_ofh.orig_sys = sys;
+            out_ofh.orig_file = outfh;
+
+            while (target_size)
+            {
+                uint blk_csize, blk_dsize, blk_crc, blk_flags;
+
+                if (sys->read(infh, buf, oabblk_SIZEOF) != oabblk_SIZEOF)
+                {
+                    ret = MSPACK_ERR_READ;
+                    goto outlbl;
+                }
+                blk_flags = EndGetI32(&buf[oabblk_Flags]);
+                blk_csize = EndGetI32(&buf[oabblk_CompSize]);
+                blk_dsize = EndGetI32(&buf[oabblk_UncompSize]);
+                blk_crc = EndGetI32(&buf[oabblk_CRC]);
+
+                if (blk_dsize > block_max || blk_dsize > target_size || blk_flags > 1)
+                {
+                    ret = MSPACK_ERR_DATAFORMAT;
+                    goto outlbl;
+                }
+
+                if (!blk_flags)
+                {
+                    /* Uncompressed block */
+                    if (blk_dsize != blk_csize)
+                    {
+                        ret = MSPACK_ERR_DATAFORMAT;
+                        goto outlbl;
+                    }
+                    ret = copy_fh(sys, infh, outfh, blk_dsize, buf, self->buf_size);
+                    if (ret) goto outlbl;
+                }
+                else
+                {
+                    /* LZX compressed block */
+                    window_bits = 17;
+
+                    while (window_bits < 25 && (1U << window_bits) < blk_dsize)
+                        window_bits++;
+
+                    in_ofh.available = blk_csize;
+                    out_ofh.crc = 0xffffffff;
+
+                    lzx = lzxd_init(&oabd_sys, (void*)&in_ofh, (void*)&out_ofh, window_bits,
+                                    0, self->buf_size, blk_dsize, 1);
+                    if (!lzx)
+                    {
+                        ret = MSPACK_ERR_NOMEMORY;
+                        goto outlbl;
+                    }
+
+                    ret = lzxd_decompress(lzx, blk_dsize);
+                    if (ret != MSPACK_ERR_OK)
+                        goto outlbl;
+
+                    lzxd_free(lzx);
+                    lzx = NULL;
+
+                    /* Consume any trailing padding bytes before the next block */
+                    ret = copy_fh(sys, infh, NULL, in_ofh.available, buf, self->buf_size);
+                    if (ret) goto outlbl;
+
+                    if (out_ofh.crc != blk_crc)
+                    {
+                        ret = MSPACK_ERR_CHECKSUM;
+                        goto outlbl;
+                    }
+                }
+                target_size -= blk_dsize;
+            }
+
+        outlbl:
+            if (lzx) lzxd_free(lzx);
+            if (outfh) sys->close(outfh);
+            if (infh) sys->close(infh);
+            sys->free(buf);
+
+            return ret;
+        }
 
         /// <summary>
         /// Decompresses an Offline Address Book with an incremental patch file.
@@ -58,8 +202,169 @@ namespace SabreTools.Compression.libmspack
         /// The filename of the output file. This is passed
         /// directly to mspack_system::open().
         /// </param>
-        /// <returns>An error code, or MSPACK_ERR_OK if successful</returns>
-        public abstract MSPACK_ERR decompress_incremental(in string input, in string @base, in string output);
+        /// <returns>An error code, or MSPACK_ERR.MSPACK_ERR_OK if successful</returns>
+        public MSPACK_ERR decompress_incremental(in string input, in string @base, in string output)
+        {
+            lzxd_stream lzx = null;
+            uint window_bits, window_size;
+            MSPACK_ERR ret = MSPACK_ERR.MSPACK_ERR_OK;
+
+            mspack_system sys = this.system;
+
+            mspack_file infh = sys.open(input, MSPACK_SYS_OPEN.MSPACK_SYS_OPEN_READ);
+            if (infh == null)
+            {
+                ret = MSPACK_ERR.MSPACK_ERR_OPEN;
+                goto outlbl;
+            }
+
+            byte[] hdrbuf = new byte[patchhead_SIZEOF];
+            byte* hdrbufPtr = libmspack.system.GetArrayPointer(hdrbuf);
+            if (sys.read(infh, hdrbufPtr, patchhead_SIZEOF) != patchhead_SIZEOF)
+            {
+                ret = MSPACK_ERR.MSPACK_ERR_READ;
+                goto outlbl;
+            }
+
+            if (EndGetI32(&hdrbuf[patchhead_VersionHi]) != 3 ||
+                EndGetI32(&hdrbuf[patchhead_VersionLo]) != 2)
+            {
+                ret = MSPACK_ERR.MSPACK_ERR_SIGNATURE;
+                goto outlbl;
+            }
+
+            uint block_max = EndGetI32(&hdrbuf[patchhead_BlockMax]);
+            uint target_size = EndGetI32(&hdrbuf[patchhead_TargetSize]);
+
+            // We use it for reading block headers too
+            if (block_max < patchblk_SIZEOF)
+                block_max = patchblk_SIZEOF;
+
+            mspack_file basefh = sys.open(@base, MSPACK_SYS_OPEN.MSPACK_SYS_OPEN_READ);
+            if (basefh == null)
+            {
+                ret = MSPACK_ERR.MSPACK_ERR_OPEN;
+                goto outlbl;
+            }
+
+            mspack_file outfh = sys.open(output, MSPACK_SYS_OPEN.MSPACK_SYS_OPEN_WRITE);
+            if (outfh == null)
+            {
+                ret = MSPACK_ERR.MSPACK_ERR_OPEN;
+                goto outlbl;
+            }
+
+            byte* buf = (byte*)sys.alloc(this.buf_size);
+            if (buf == null)
+            {
+                ret = MSPACK_ERR.MSPACK_ERR_NOMEMORY;
+                goto outlbl;
+            }
+
+            mspack_oab_system oabd_sys = sys as mspack_oab_system;
+
+            oabd_file in_ofh = new oabd_file();
+            in_ofh.orig_sys = sys;
+            in_ofh.orig_file = infh;
+
+            oabd_file out_ofh = new oabd_file();
+            out_ofh.orig_sys = sys;
+            out_ofh.orig_file = outfh;
+
+            while (target_size > 0)
+            {
+                if (sys.read(infh, buf, patchblk_SIZEOF) != patchblk_SIZEOF)
+                {
+                    ret = MSPACK_ERR.MSPACK_ERR_READ;
+                    goto outlbl;
+                }
+
+                uint blk_csize = EndGetI32(&buf[patchblk_PatchSize]);
+                uint blk_dsize = EndGetI32(&buf[patchblk_TargetSize]);
+                uint blk_ssize = EndGetI32(&buf[patchblk_SourceSize]);
+                uint blk_crc = EndGetI32(&buf[patchblk_CRC]);
+
+                if (blk_dsize > block_max || blk_dsize > target_size ||
+                    blk_ssize > block_max)
+                {
+                    ret = MSPACK_ERR.MSPACK_ERR_DATAFORMAT;
+                    goto outlbl;
+                }
+
+
+                window_size = (blk_ssize + 32767) & ~32767;
+                window_size += blk_dsize;
+                window_bits = 17;
+
+                while (window_bits < 25 && (1U << window_bits) < window_size)
+                    window_bits++;
+
+                in_ofh.available = blk_csize;
+                out_ofh.crc = 0xffffffff;
+
+                lzx = lzxd_init(&oabd_sys, (void*)&in_ofh, (void*)&out_ofh, window_bits,
+                                0, 4096, blk_dsize, 1);
+                if (!lzx)
+                {
+                    ret = MSPACK_ERR.MSPACK_ERR_NOMEMORY;
+                    goto outlbl;
+                }
+                ret = lzxd_set_reference_data(lzx, sys, basefh, blk_ssize);
+                if (ret != MSPACK_ERR.MSPACK_ERR_OK)
+                    goto outlbl;
+
+                ret = lzxd_decompress(lzx, blk_dsize);
+                if (ret != MSPACK_ERR.MSPACK_ERR_OK)
+                    goto outlbl;
+
+                lzxd_free(lzx);
+                lzx = null;
+
+                /* Consume any trailing padding bytes before the next block */
+                ret = copy_fh(sys, infh, null, in_ofh.available, buf, this.buf_size);
+                if (ret) goto outlbl;
+
+                if (out_ofh.crc != blk_crc)
+                {
+                    ret = MSPACK_ERR.MSPACK_ERR_CHECKSUM;
+                    goto outlbl;
+                }
+
+                target_size -= blk_dsize;
+            }
+
+        outlbl:
+            if (lzx) lzxd_free(lzx);
+            if (outfh) sys.close(outfh);
+            if (basefh) sys.close(basefh);
+            if (infh) sys.close(infh);
+            sys.free(buf);
+
+            return ret;
+        }
+
+        private static MSPACK_ERR copy_fh(mspack_system sys, mspack_file infh, mspack_file outfh, int bytes_to_copy, byte* buf, int buf_size)
+        {
+            while (bytes_to_copy > 0)
+            {
+                int run = buf_size;
+                if (run > bytes_to_copy)
+                {
+                    run = bytes_to_copy;
+                }
+                if (sys.read(infh, buf, run) != run)
+                {
+                    return MSPACK_ERR.MSPACK_ERR_READ;
+                }
+                if (outfh != null && sys.write(outfh, buf, run) != run)
+                {
+                    return MSPACK_ERR.MSPACK_ERR_WRITE;
+                }
+                bytes_to_copy -= run;
+            }
+
+            return MSPACK_ERR.MSPACK_ERR_OK;
+        }
 
         /// <summary>
         /// Sets an OAB decompression engine parameter. Available only in OAB
@@ -72,9 +377,19 @@ namespace SabreTools.Compression.libmspack
         /// <param name="param">The parameter to set</param>
         /// <param name="value">The value to set the parameter to</param>
         /// <returns>
-        /// MSPACK_ERR_OK if all is OK, or MSPACK_ERR_ARGS if there
+        /// MSPACK_ERR.MSPACK_ERR_OK if all is OK, or MSPACK_ERR.MSPACK_ERR_ARGS if there
         /// is a problem with either parameter or value.
         /// </returns>
-        public abstract MSPACK_ERR set_param(MSOABD_PARAM param, int value);
+        public MSPACK_ERR set_param(MSOABD_PARAM param, int value)
+        {
+            if (param == MSOABD_PARAM.MSOABD_PARAM_DECOMPBUF && value >= 16)
+            {
+                // Must be at least 16 bytes (patchblk_SIZEOF, oabblk_SIZEOF)
+                this.buf_size = value;
+                return MSPACK_ERR.MSPACK_ERR_OK;
+            }
+
+            return MSPACK_ERR.MSPACK_ERR_ARGS;
+        }
     }
 }
\ No newline at end of file
diff --git a/libmspack/mspack_default_system.cs b/libmspack/mspack_default_system.cs
index 0b70577..d33d37a 100644
--- a/libmspack/mspack_default_system.cs
+++ b/libmspack/mspack_default_system.cs
@@ -120,8 +120,7 @@ namespace SabreTools.Compression.libmspack
         /// <inheritdoc/>
         public override unsafe void* alloc(int bytes)
         {
-            var arr = new byte[bytes];
-            return (byte*)arr[0];
+            return system.CreateArray<byte>(bytes);
         }
 
         /// <inheritdoc/>
diff --git a/libmspack/mspack_oab_system.cs b/libmspack/mspack_oab_system.cs
new file mode 100644
index 0000000..fd0c187
--- /dev/null
+++ b/libmspack/mspack_oab_system.cs
@@ -0,0 +1,34 @@
+namespace SabreTools.Compression.libmspack
+{
+    public unsafe class mspack_oab_system : mspack_default_system
+    {
+        /// <inheritdoc/>
+        public override unsafe int read(mspack_file base_file, void* buf, int size)
+        {
+            oabd_file file = (oabd_file)base_file;
+            int bytes_read;
+
+            if (size > file.available)
+                size = file.available;
+
+            bytes_read = file.orig_sys.read(file.orig_file, buf, size);
+            if (bytes_read < 0)
+                return bytes_read;
+
+            file.available -= bytes_read;
+            return bytes_read;
+        }
+
+        /// <inheritdoc/>
+        public override unsafe int write(mspack_file base_file, void* buf, int size)
+        {
+            oabd_file file = (oabd_file)base_file;
+            int bytes_written = file.orig_sys.write(file.orig_file, buf, size);
+
+            if (bytes_written > 0)
+                file.crc = mspack.crc32(file.crc, buf, bytes_written);
+
+            return bytes_written;
+        }
+    }
+}
\ No newline at end of file
diff --git a/libmspack/oabd_file.cs b/libmspack/oabd_file.cs
new file mode 100644
index 0000000..4a838b9
--- /dev/null
+++ b/libmspack/oabd_file.cs
@@ -0,0 +1,13 @@
+namespace SabreTools.Compression.libmspack
+{
+    public class oabd_file : mspack_file
+    {
+        public mspack_system orig_sys { get; set; }
+
+        public mspack_file orig_file { get; set; }
+
+        public uint crc { get; set; }
+
+        public int available { get; set; }
+    }
+}
\ No newline at end of file