avra: begin writing docs

blk/201

@@ -3,7 +3,7 @@ Forth words, opcode assembly is a bit different than with a
 typical assembler. For example, what would traditionally be
 "ld a, b" would become "A B LDrr,".
 
-The "argtype" prefix after each mnemonic is needed because the
+The "argtype" suffix after each mnemonic is needed because the
 assembler doesn't auto-detect the op's form based on arguments.
 It has to be explicitly specified. "r" is for 8-bit registers,
 "d" for 16-bit ones, "i" for immediate, "c" is for conditions.
@@ -12,5 +12,5 @@ bit ops can affect SP, others, AF. If you use the wrong argu-
 ment on the wrong op, you will affect the wrong register.
 
 Mnemonics having only a single form, such as PUSH and POP,
-don't have argtype prefixes.
+don't have argtype suffixes.
                                                         (cont.)

blk/650

@@ -1 +1,16 @@
 AVR assembler
+
+This assembler works very much like Z80 assembler (B200) so 
+refer to this documentation first. Here, we document specifici-
+ties.
+
+All mnemonics in AVR have a single signature. Therefore, we     
+don't need any "argtype" suffixes. 
+
+Registers are referred to with consts R0-R31. There is
+X, Y, Z, X+, Y+, Z+, X-, Y-, Z- for appropriate ops (LD, ST).
+XL, XH, YL, YH, ZL, ZH are simple aliases to R26-R31.
+
+Branching works differently. Instead of expecting a byte to be  
+written after the naked op, branching words expect a displace-  
+ment argument.                                           (cont.)

blk/651

@@ -0,0 +1,9 @@
+This is because there's bitwise ORing involved in the creation  
+of the final opcode, which makes z80a's approach impractical.
+
+This makes labelling a bit different too. Instead of expecting  
+label words after the naked branching op, we rather have label
+words expecting branching wordref as an argument. Examples:
+
+L2 ' BRTS FLBL! ( branch forward to L2 )
+L1 ' RJMP LBL, ( branch backward to L1 )