The M register in the HP-65 is used for a lot of very interesting things. Maybe not quite “give me the contents of the M register and I’ll show you the world”; but it is very close to that. <\/p>\n
The M register has, of course, 14 digits. The digits are numbered by HP from right to left starting with 0 so we have:
\nM= 13 12 11 10 9 8 7 6 5 4 3 2 1 0<\/p>\n
The three rightmost [2,1,0] are the exponent (“[x]” in the microcode) including [2], the exponent sign (“[xs]” in the microcode).<\/p>\n
The other digits are [13] – the mantissa sign (“[ms]”) and [12,…,3] – the mantissa (“[m]”).<\/p>\n
When you copy the contents of the M register into the C register individual digits are accessible via the “c[p]” construct in microcode instructions.<\/p>\n
Okay, enough of the basis. Here’s what is in it:
\nM[13]: ???
\nM[12]: 1= DSP was pressed, 2= DSP . pressed
\nM[11]: 1= GTO was pressed.
\nM[10]: 1= LBL was pressed.
\nM[ 9]: ???
\nM[ 8]: ???
\nM[ 7]: 1= f was pressed.
\nM[ 6]: 1= f-1 was pressed.
\nM[ 5]: 1= STO was pressed, 2-5= STO {-+*\/} pressed.
\nM[ 4]: 1= RCL was pressed.
\nM[ 3]: 1= g was pressed.
\nM[ 2]: precision (2 if DSP . 2, 5 if DSP 5)
\nM[ 1]: mode (0=SCI, 2=FIX)
\nM[ 0]: angle mode (0=RAD, 1=DEG, 2=GRD)<\/p>\n
There is an elegant simplicity to it. The content looks a lot like what we see in RAM[62] of a HP-67 although a lot of that is blank as the ’67 tracks prefix keys through a different mechanism.<\/p>\n","protected":false},"excerpt":{"rendered":"
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