S Register Flags
================

s0 = key pressed
s1 = input_started (number keys or EEX or dot, have been pressed)

s3 = w/prgm (0=in run mode)
s5 = (in run mode) error (1 causes flashing display)
s5 = (in w/prgm mode) prgm mem full (1 causes trailing minus)
s8 = running (1= get key from prgm mem)
s9 = display_input (show what has been keyed in)

s10= dont-interupt. Ignores keypress during running program.
s11= last f flag went from 1->0

s2 = (input) dot_pressed
s7 = (input) eex_pressed

s4 = ?
s6 = ?


F Flags
=======

f1 = auto_enter. data entry lifts stack. 0 after CLx and ENTER.
f5 = ready (0 when prgm mem busy)

f3 = display_x (0= skip the display process, A & B are already set. 1= show stack X (C register value))
f4 = f SF 1
f6 = f SF 2

f7 = special mode. (key->prgm mem, computed gotos, ...)

f0 = ?
f2 = step100. "-     35 01-  ". Set if prg pointer is after (at) step 100


M Register Flags
================

M[13]: 1= in main routine, 0= in sub routine
M[12]: 1= DSP was pressed, 2= DSP . pressed
M[11]: 1= GTO was pressed.
M[10]: 1= LBL was pressed.
M[ 9]: ???
M[ 8]: ???
M[ 7]: 1= f was pressed.
M[ 6]: 1= f-1 was pressed.
M[ 5]: 1= STO was pressed, 2-5= STO {-+*/} pressed.
M[ 4]: 1= RCL was pressed.
M[ 3]: 1= g was pressed.
M[ 2]: precision (2 if DSP . 2, 5 if DSP 5)
M[ 1]: mode (0=SCI, 2=FIX)
M[ 0]: angle mode (0=RAD, 1=DEG, 2=GRD)


Key Parts of the Microcode
==========================

00000 Power up

00726 - 00777. The wait loop.
00726 waitloop1
00622 waitloop2 (checks for interrupt)
00764 waitloop3
00774 waitloop4

00625 a key has been pressed
01316 switch (key) - uses 01000 jump table

00531 display
00554 setup A & B for display
00616 display X in SCI notation
04164 display X in FIX notation

02127 MCIRC0 - subroutine to wait for prgm mem to be ready (last action to complete)

00750 if running get next prgm step