Move AVR assembler from B650 to B50

And move its doc to doc/asm.txt

blk/001

@@ -1,13 +1,13 @@
 MASTER INDEX
 
 005 Z80 assembler             030 8086 assembler
-50-99 unused
+050 AVR assembler             70-99 unused
 100 Block editor              120 Visual Editor
 160-259 unused                260 Cross compilation
 280 Z80 boot code             350 Core words
 410 PS/2 keyboard subsystem   420 SD Card subsystem
 440-519 unused                520 Fonts
-550-649 unused
-650 AVR assembler             690 AVR SPI programmer
+550-689 unused
+690 AVR SPI programmer
 700-799 unused
 800 8086 boot code

doc/asm.txt

@@ -159,3 +159,68 @@ For jumps, it's special. 's' is SHORT, 'n' is NEAR, 'f' is FAR.
 # 8086 Instructions list
 
 TODO
+
+# AVR assembler
+
+Load with "50 LOAD". As with the Z80 assembler, it is incom-
+plete.
+
+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.
+
+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 )
+
+# AVR instructions list
+
+OPRd (B53)
+ASR  COM   DEC   INC   LAC   LAS   LAT   LSR   NEG   POP   PUSH
+ROR  SWAP  XCH
+
+OPRdRr (B54)
+ADC  ADD   AND   CP    CPC   CPSE  EOR   MOV   MUL   OR   SBC
+SUB
+
+OPRdA (B54)
+IN   OUT
+
+OPRdK (B55)
+ANDI CPI   LDI   ORI   SBCI   SBR   SUBI
+
+OPAb (B55)
+CBI   SBI   SBIC  SBIS
+
+OPNA (B56)
+BREAK  CL[C,H,I,N,S,T,V,Z] SE[C,H,I,N,S,T,V,Z] EIJMP ICALL
+EICALL IJMP  NOP   RET   RETI  SLEEP WDR
+
+OPb (B57)
+BCLR  BSET
+
+OPRdb (B57)
+BLD   BST   SBRC  SBRS
+
+Special (B57,B60)
+CLR   TST   LSL   LD    ST
+
+Flow (B58)
+RJMP  RCALL
+BR[BC,BS,CC,CS,EQ,NE,GE,HC,HS,ID,IE,LO,LT,MI,PL,SH,TC,TS,VC,VS]
+
+Flow macros (B61)
+LBL! LBL, SKIP, TO, FLBL, FLBL! BEGIN, AGAIN? AGAIN, IF, THEN,