add a few porting comments

Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@2915

docs/PORTING

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+Some notes about porting DOSBox to systems with certain restrictions,
+like handheld devices.
+
+If memory is a constraint:
+  - in paging.h reduce the size of the TLB:
+      #define TLB_SIZE (64*1024)
+    the 64 gives access to the first 256mb of linear memory
+    drawback: some protected mode games won't work then
+    gain: reduces memory requirements about ~15mb
+    TODO: limit vmemory access to the size of the TLB,
+          otherwise games which use >256mb vaddresses will crash badly
+  - in render.h lower the scaler integration:
+      #define RENDER_USE_ADVANCED_SCALERS 1
+    or
+      #define RENDER_USE_ADVANCED_SCALERS 0
+    drawback: complex scalers and the scaler cache are disabled,
+              be sure to test if this affects speed!
+              with define RENDER_USE_ADVANCED_SCALERS==0 most simple
+              scalers are disabled as well, some graphics modes won't
+              work due to reduced cache sizes
+    gain: ~2mb with RENDER_USE_ADVANCED_SCALERS==1
+          ~5mb with RENDER_USE_ADVANCED_SCALERS==0
+  - in dos_system.h reduce the drive cache entries:
+      #define MAX_OPENDIRS 256
+    drawback: some apps might not work whith large directory trees
+    gain: ~1mb per mounted drive
+  - in vga.h reduce the size of the emulated graphics memory:
+      #define VGA_MEMORY (1*1024*1024)
+    drawback: some graphics modes won't work then
+    gain: reduces memory requirements about 1mb
+    TODO: remove the respective svga modes, adapt some svga registers
+  - remove the GUS emulation (gus.cpp, especially GUSRam[1024*1024] )
+    drawback: no gravis ultrasound
+    gain: reduces memory requirements about 1mb
+
+If speed is a constraint:
+  - see if the simple core is faster, possibly remove the normal core
+    set the simple core as default
+    drawback: one game is known to not work with the simple core;
+              the simple core does only work for games which don't use paging
+              (when paging is requested the normal core is used automatically)
+    gain: the simple core should be somewhat faster
+    TODO: add possibility to easily remove the normal core, use fullcore fallback
+  - raise the default frameskip value
+    drawback: minor graphics smoothness loss for some games (video playback)
+    gain: reduces graphics load