Add a description of how immediate actions should be encoded.

Also sets the action list type to unsigned short (uint16_t) which
I think is the most appropriate type for s390x (x86 uses uint8_t
and other platforms use uint32_t).

dynasm/dasm_s390x.h

@@ -9,7 +9,7 @@
 #include <string.h>
 #include <stdlib.h>
 
-#define DASM_ARCH		"s390"
+#define DASM_ARCH		"s390x"
 
 #ifndef DASM_EXTERN
 #define DASM_EXTERN(a,b,c,d)	0
@@ -49,7 +49,7 @@ enum {
 #define DASM_POS2PTR(D, pos)	(D->sections[DASM_POS2SEC(pos)].rbuf + (pos))
 
 /* Action list type. */
-typedef const unsigned int *dasm_ActList;
+typedef const unsigned short *dasm_ActList;
 
 /* Per-section structure. */
 typedef struct dasm_Section {

dynasm/dasm_s390x.lua

@@ -970,6 +970,19 @@ local function parse_template(params, template, nparams, pos)
 
     end
   end
+
+  -- TODO
+  -- 12-bit displacements (DISP12) and 16-bit immediates (IMM16) can be put at
+  -- one of two locations relative to the end of the instruction.
+  -- To make decoding easier we should insert the actions for these immediately
+  -- after the halfword they modify.
+  -- For example, take the instruction ahik, which is laid out as follows (each
+  -- char is 4 bits):
+  -- o = op code, r = register, i = immediate
+  -- oorr iiii 00oo
+  -- This should be emitted as oorr, followed by the immediate action, followed by
+  -- 00oo.
+
   wputpos(pos, op)
 end
 function op_template(params, template, nparams)