15. Instruction sets

Both the HP and Jacobs-DeArras instruction sets are supported. The tools will use HP by default.

The reason for that there are two popular instruction sets is that HP did not originally publish its instruction set. Based on available fragments of information and a tremendous work by the user community, the instruction set was figured out.

Jim DeArras even went on to document the disassembled listings of the HP-41 firmware and when HP realized that the community was succeeding, they released the original listings.

However, the momentum was ongoing and we ended up with two popular instruction sets.

Why choose one over the other? The main reason to choose the HP instruction set is that it makes it easier to read the available source code listings. The main reason to choose the Jacobs-DeArras instruction set is that it is very popular.

No matter which you choose, you will most likely end up having a decent knowledge of the other instruction set as well. This section will take you through the main differences.

15.1. Non-local jumps

The 64K conditional jump and jump to subroutine instructions are named differently.

Table 15.1 Non-local jumps

HP	Jacobs-DeArras
gosub	?ncxq
gsubc	?cxq
golong	?ncgo
golc	?cgo

HP uses variants gsubnc and golnc which typically used after a test instruction (that sets carry) to say that we do care about the carry here not to be set. The gosub and golong variants are used when we know the carry will be clear and the instruction is meant to be unconditional (the carry auto-resets after each instruction that does otherwise affect it).

15.2. Local branches

Short branches also have different names.

Table 15.2 Local branches

HP	Jacobs-DeArras
goto	jnc
goc	jc

Similar to non-local jumps, the gonc instruction is used after a test when we want to make it clear that we branch on carry not set.

15.3. Other jump and subroutine instructions

Table 15.3 Subroutine related

HP	Jacobs-DeArras
gotoc	gotoadr
stk=c	pushadr
c=stk	popadr
spopnd	xq>go
rtnnc	?ncrtn
rtnc	?crtn

15.4. Page relocatable jumps

Table 15.4 Page relative jumps

HP	Jacobs-DeArras
gsbp	rxq
golpc	rgo
gsb41c	rxq
gol41c	rgo

15.5. Pointer instructions

Table 15.5 Pointer related

HP	Jacobs-DeArras
selp	slctp
selq	slctq
decpt	r=r-1
incpt	r=r+1
pt=	r=
?pt=	?r=

15.6. Arithmetic instructions

These are mostly the same in both the instruction sets. There are a couple of differences as follows:

Table 15.6 Arithmetics

HP	Jacobs-DeArras
abex	a<>b
acex	a<>c
bcex	c<>b
c=-c	c=0-c

HP favors alphabetic order in the exchange instructions, while Jacobs-DeArras favors the more versatile register first.

The operand field to the arithmetic instructions shows more variation:

Table 15.7 Arithmetic operand fields

HP	Jacobs-DeArras
pt	@r
x	s&x
wpt	r<-
w	all
pq	p-q
xs	xs
m	m
s	ms

HP has the benefit of using only letters, which makes it easier to parse them as register fields in expressions (used in the debugger).

15.7. Flag instructions

This is a significant areas where the instruction sets differ. Also note that st=0 exists in both instruction sets, but are used for different instructions.

Table 15.8 Flags

HP	Jacobs-DeArras
st=0	crlf
s5=0 etc	clrf  5
st=1	setf
s5=1 etc	setf  5
?st=1	?fset
st=1?	?fset
?s5=1 etc	?fset 5
clrst	st=0
cstex	c<>st

15.8. Data memory

This is another area of significant differences in naming.

Table 15.9 Data memory

HP	Jacobs-DeArras
dadd=c	ramslct
data=c	writedata
c=data	readdata
regn=c	writ
c=regn	read

15.9. Keyboard

Table 15.10 Keyboard

HP	Jacobs-DeArras
rstkb	clrkey
chkkb	?key
c=keys	c=key
gokeys	gtokey

15.10. Miscellaneous

Here are the remaining instructions that differs.

Table 15.11 Miscellaneous

HP	Jacobs-DeArras
cxisa	fetch
lc	ld@r
selpf	selp
?flg=1	?fi
cnex	c<>n
cmex	c<>m
cgex	c<>g
f=sb	t=st
sb=f	st=t
fexsb	st<>t
?lld	?lowbat
clrabc	a=b=c=0
pfad=c	prphslct
disoff	dspoff
distog	dsptog