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	hp41c.c		HP41C calculator emulator	v1.4
					Harold Z. Bencowitz
					Beaumont, Texas
					24-sep-86

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description:

	hp41c is a program to emulate the Hewlett-Packard 41C series of
	hand-held programmable calculators. it requires a vt100 or vt200
	series terminal. it has been tested on rt11 v5.3 and tsx+ v6.01.
	the major design goal was to make an exactly identical user
	interface for those features included. to operate the "calculator"
	follow directions in the HP41C owners handbook. the keyboard
	mapping is given on screen when the program is run. many features of
	the hp41c are not supported, primarily programming and functions
	which are only useful from programs.

features supported:

	display control functions ie <-, CLX, EEX, FIX, ENTER, and CHS;
	all mathematical functions available from the keyboard (without
	XEQ) namely: add, subtract, multiply, divide, 1/x, x**2, SQRT,
	y**x, ln, e**x, log, and 10**x; OCT and DEC (decimal/octal
	conversions (both from keypad and XEQ); rotate stack down (RDN);
	trig functions (SIN, COS, TAN, ASIN, ACOS, ATAN); nonvolatile
	memory (if program is named sy:hp41c.sav); degree/radian/grad mode
	switching for trig functions; STO and RCL (store and recall from
	memory); storage register arithmetic, direct storage in stack
	registers; XEQ commands D-R, DEC, DEG, RAD, FRC, GRAD, INT, CLRG,
	OCT, PI, MOD, CLST, R-D.

features not supported:

	alpha mode (including AON, AOFF, ASHF, ASTO, AVIEW, CLA), user
	defined keys (user mode), indirect memory access, last x, LN1+X,
	accumulations (CLsum, sum+, sum-, sumREG, mean, sum, SD), BEEP,
	TONE, VIEW, ADV, SIZE, COPY, ON, OFF, rotate up, programming
	(including numerous functions such as R/S, pack, catalog, sst,
	bst, CLP, DEL, DSE, END, all flag instructions, GTO, GTO., GTO..,
	ISG, LBL, PROMPT, PSE, RTN, STOP, and all comparisons), and 
	some xeq commands including: ABS, ENG, SCI, FACT, HMS, HMS+, HMS-,
	HR, LASTX, %, %CH, P-R, R-P, RND, SIGN (some of these are only
	useful from programs).
	
operating instructions:

	remember that the HP41C uses reverse polish notation.
	to operate the "calculator" follow directions in the HP41C
	owners handbook. almost all operations are the same when
	supported although many HP41C features are not supported.
	the mapping of the terminal keyboard to the "calculator"
	is explained on screen. the set of functions on the right
	are those activated by pressing the gold key prior to the
	specific key. almost all "calculator" functions
	are operated from the keypad. in addition four keys are
	used which are F17-F20 on a vt200 and UP-RIGHT (arrows) on
	a vt100. in addition the delete key on each terminal turns
	the program off (control C is not disabled but it should be
	avoided, otherwise the vt200 will be left with the cursor
	off and the current data will not be written to disk). only
	these keys are active and others are ignored. the
	functions available through the XEQ command are listed on
	the far left of the screen. XEQ does not require turning
	alpha mode on as the HP41C does. pressing XEQ automatically
	enables alpha mode. to terminate an alpha string
	press "." on the keypad. during alpha mode only A-Z (lowercase
	converted to upper), "-", keypad comma (<- function), and keypad
	period (terminate alpha mode) are active.

possible additions:

	accumulator functions (+ACC, -ACC, mean, SDEV, and clear acc),
	and some of the following functions using XEQ: ABS, CLD, ENG,
	SCI, FACT, HMS, HMS+, HMS-, HR, LASTX, %, %CH, P-R, R^, R-P,
	RND.

differences/limitations:

	uses standard PDP11 double floats in which the permissible size
	of a number is ~1.7e-38 to ~1.7e38 whereas the HP41C allows
	9.9...e-99 to 9.9...e99. the program will not allow
	any value outside the range 1.0e-37 to 1.0e37. the program
	deals with out of range entries (NOT out of range results)
	differently than the HP41C. it displays an error message and
	remains in active mode allowing revision of the entered number,
	whereas the HP41C merely truncates the number to 0. or 9.9...e99
	and tries to use it.

	PDP11 double floats have more decimal places accuracy than the
	HP41C. this is corrected for in the software. however occasionally
	this difference will result in a slightly different result than
	the HP41C (eg 1.123 - 1, then the result - .123 = a very small
	number rather than zero).

	DEC and OCT functions are available through keyboard commands
	as well as XEQ.

	XEQ does not require turning alpha mode off and on as the HP41C
	does. pressing XEQ automatically enables alpha mode. to terminate
	an alpha string press "." on the keypad.

	the trig functions may give slightly different results (7-10th
	decimal place). some trig functions may occasionally give very
	small answers instead of 0. (eg. tan of 720 degrees) or vice
	versa (sine of -1.236e-20).

	no other differences are known.


revision history:

	v1.0 completed 24-jun-86	includes all display control keys
					includes + - / * and 1/x

	v1.1 completed 14-jul-86	keys added to displayed map,
					error in vt100 active,
					"shift" display, cutoff,
					x**2, sqrt, y**x, ln, e**x,
					log, 10**x, oct, dec

	v1.2 completed 18-jul-86	RDN, DEG/RAD/GRAD mode switching,
					STO, RCL, delete from anywhere
					to stop

	v1.3 completed 21-jul-86	XEQ (RAD, DEC, DEG, GRAD, CLST,
					CLRG, OCT, PI), storage in stack
					nonvolatile memory (write to disk)

	v1.4 completed 24-sep-86	fix bugs in k_oct and k_dec;
					set_inactive() created,
					improved general purpose isint();
					SIN, COS, TAN, ASIN, ACOS, ATAN;
					XEQ: INT, FRC, MOD. vtttid() modified
					to allow vt132 and vt100 with printer
					port.

installation/building:

	included: hp41c.sav, hp41c.h, hp41c.c, vt.obj, hclib.obj, hp41c.end,
	and hp41c.doc (included in hp41c.c also). to install: copy
	hp41c.sav to sy:. it will run from other locations but error
	messages will appear whenever the program is run or stopped if
	it cannot read/write from sy:hp41c.sav. 

	the source module hp41c.c must be compiled and linked with library
	modules from hclib (my own c function library), clib (whitesmith's
	c function library), vt (my video terminal library), and syslib.
	after compilation and linking, the size of hp41c.sav must equal the
	number of blocks defined by BLOCK below. if not, change BLOCK and
	recompile. otherwise, append hp41c.end to hp41.sav to make the final
	hp41c.sav.

definition:

	active mode means that entry is one key at a time prompted by
	'_' (if not in the far right character position) where entries
	can be edited with <-. inactive mode numbers cannot be edited.

implementation notes:

	there is one source file hp41c.c and a header file hp41c.h.
	the video terminal library vt.obj, my library hclib.obj, and
	Whitesmith's clib.obj are also required.

	the program queries the terminal for type. it will not run
	unless the identification indicates a vt100 series or vt200
	(in 7 bit mode only) series terminal. the following are
	differences between vt100 and vt200 implementations:
	1) downloadable character sets on the vt200 allow the display
	to place the decimal point "between" two numbers rather than
	as a separate character and 2) the cursor is turned off on a
	vt200. the program uses special terminal mode and thus requires
	running in singlechar mode with TSX+ (done automatically)
	and changes the keypad and cursor control (arrow) keys to
	application mode. all keyboard input is captured and undefined
	keys are ignored. control-C is not disabled.

	the active display is defined by adc[], expdc[3], adc_sign,
	adc_dec, adc_dec, adc_dot, exp_flag, exp_sign, and exp_end.
	this is explained in comments in active(). the calculator
	stack is in extern double stack[3] (T, Z, and Y
	are in [2], [1], and [0] respectively) and x_value (X). dis_fix
	is the number of decimal places to display as entered by FIX.
	note that this is different from automatic variable fix.
	act_flag is YES if the display is in active mode. err_flag is
	YES if an error message is displayed (set NO by display(), set
	YES by derror(), only used by k_clc). lift_stack if YES means
	that the next time the display changes from inactive to active,
	the x value will be pushed onto the stack. this is explained in
	appendix C of the HP41C manual. stack lift is disabled by
	ENTER and CLX. it is not changed by <-. CHS and EEX do not
	change it during active mode; at other times they enable stack
	lift. all other operations enable stack lift. 

	to use non-volatile memory, ie read/write data to disk, append
	2 blocks to the compiled and linked *.sav image. the first word
	of the two blocks must be CALCODE. BLOCK, the starting block number
	to read must be equal to the length of the original *.sav file
	in blocks. the easiest way to do this is to append hp41c.end.
	this feature is turned off at compilation if NVMEM is undefined.