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julie_m · 8th Jun 2019, 11:44 pm

As part of a program which involves converting decimal numbers to binary, I wrote a dedicated subroutine to multiply by ten, in 16-bit unsigned arithmetic. Ten is known as a sparse number, because its binary representation contains only a few ones (just 2 i,n fact: 1010.) This means we can do our multiplication as follows:

Make a copy
Double the original number
Double it again (now we have n * 4)
Add the copy (giving n * 5)
Double it one last time.

This will be quicker than a "general-purpose" multiplying routine, which would have to check every bit in the multiplier.

Code:

.times10
LDX #0
JSR cpydn
JSR dbldn
JSR dbldn
LDX #0
CLC
JSR add_dn
.dbldn
ASL decnum
ROL decnum+1
RTS
.cpydn
JSR cpydn_1
.cpydn_1
LDA decnum,X
STA dncpy,X
INX
RTS
.add_dn
JSR add_dn1
.add_dn1
LDA decnum,X
ADC dncpy,X
STA decnum,X
INX
RTS
\TEMPORARY WORKSPACES
.decnum EQUW 0
.dncpy EQUW 0

decnum and decnum + 1 are used to store the decimal number which gets multiplied by 10 in situ, and dncpy and dncpy + 1 are used to store a copy of the original value during the multiplication. X gets stomped on, and Z=0 on exit (so a BNE instruction following a JSR here will always branch). The code itself isn't relocatable, as it contains absolute jumps.

8th Jun 2019, 11:44 pm	#13
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Fun with 6502 Assembler As part of a program which involves converting decimal numbers to binary, I wrote a dedicated subroutine to multiply by ten, in 16-bit unsigned arithmetic. Ten is known as a sparse number, because its binary representation contains only a few ones (just 2 i,n fact: 1010.) This means we can do our multiplication as follows: Make a copy Double the original number Double it again (now we have n * 4) Add the copy (giving n * 5) Double it one last time. This will be quicker than a "general-purpose" multiplying routine, which would have to check every bit in the multiplier. Code: .times10 LDX #0 JSR cpydn JSR dbldn JSR dbldn LDX #0 CLC JSR add_dn .dbldn ASL decnum ROL decnum+1 RTS .cpydn JSR cpydn_1 .cpydn_1 LDA decnum,X STA dncpy,X INX RTS .add_dn JSR add_dn1 .add_dn1 LDA decnum,X ADC dncpy,X STA decnum,X INX RTS \TEMPORARY WORKSPACES .decnum EQUW 0 .dncpy EQUW 0 decnum and decnum + 1 are used to store the decimal number which gets multiplied by 10 in situ, and dncpy and dncpy + 1 are used to store a copy of the original value during the multiplication. X gets stomped on, and Z=0 on exit (so a BNE instruction following a JSR here will always branch). The code itself isn't relocatable, as it contains absolute jumps. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.