This file contains information on some undocumented features of Intel's 8085
/80C85 microprocessor.
   WARNING: user beware! Intel has never documented these, and has NO 
obligation to support them. Always back up important files before experimenting
with this (or any other) ML stuff.

   Additions, corrections, amusing anecdotes, etc. most welcome!
   Tim Peters  72227,2416


Will It Work on My Machine?
---------------------------
   The "features" below have worked on ALL Model 100's tested to date, & I've 
heard that they're also used in some commercial software.
   Following is a short BASIC program (checksum 7682) that tests 3 of the "new"
opcodes:

1 A%(0)=1576:A%(1)=-9968:A%(2)=201
2 V=VARPTR(A%(0)):CALL V,0,V
3 IF A%(3)=V\2-(V AND 1) THEN PRINT"OK!" ELSE BEEP:PRINT">> Ouch!! <<"

   A number of things MAY happen when you run this:
 (1) it will probably print "OK!".
 (2) may beep & print "Ouch!!".
 (3) the power may go off (some 8085's may trigger the TRAP interrupt on the 
"unused" opcodes; that interrupt points to power-off code in the M100).
 (4) anything else, up to & including a coldstart!
   #1 (prints "OK!") is the ONLY response I've heard of on the M100 - if your 
machine does something else, PLEASE, let me know!


New Flags
---------
   In the table below, bit 7 is the most-significant bit; "x" marks an 
undocumented flag bit:

 0  Carry
 1x Overflow
 2  Parity even
 3  (unused)
 4  Half-carry
 5x "True" sign = Overflow XOR Sign
 6  Zero
 7  Sign

   The "overflow" flag is ordinary: it's set when the carry into the sign bit 
does not equal the carry out of the sign bit; if set, it means that the result 
of an operation on two signed bytes was "too big" to fit in one signed byte.
   Bit 5 gives the "true" sign, assuming that the operands are signed bytes; 
it's what would be in bit 8 IF the CPU were doing 9-bit signed arithmetic.


New Opcodes
-----------
   Details of the ten undocumented opcodes; the opcodes are given in hex:

 08 (1 byte) HL=HL-BC.
 10 (1 byte) Arithmetic right shift: shifts HL right one bit, preserving H's 
bit 7 (ie, sign-extended shift).
 18 (1 byte) Rotates DE left thru carry: carry flag moves to E's bit 0, and D's
bit 7 moves to the carry flag.
 28 (2 bytes) DE=HL+following (unsigned) byte
 38 (2 bytes) DE=SP+following (unsigned) byte
 CB (1 byte) CALL 40H if overflow flag set. Useless for the M100 unless you're 
writing ROM routines - awfully clumsy even then!
 D9 (1 byte) (DE)=HL; stores HL at the address in DE.
 ED (1 byte) HL=(DE); loads HL from the address in DE.
 DD (3 bytes) Jump (to address in following two bytes) if flag bit 5 is clear.
 FD (3 bytes) Jump (to address in following two bytes) if flag bit 5 is set.