The MULTIPLY function for Diamond GOS is stated as:

MULTIPLY (39) - Word sized multiply(please note overflow error are not detected)
Receives:
W5 - Multiplicand 1
W6 - Multiplicand 2
Returns:
W7 - Result

As with the DIVIDE macro, the original macro received the numbers when calling it. It was changed to accept an address of the numbers and the numbers are then copied into W5 and W6.

Original macro
8500 ;
8510 .MACRO MULTIPLY
8520 LDA # <%1 ;MULTIPLICAND 1
8530 STA W5
8540 LDA # >%1
8550 STA W5+1
8560 LDA # <%2 ;MULTIPLICAND 2
8570 STA W6
8580 LDA # >%2
8590 STA W6+1
8600 LDA #MULTIPLY
8610 JSR DIAMOND
8620 .ENDM
8630 ;

Modified macro
8500 ;
8505 ;MODIFIED TO LOAD NUM FROM ADDR.
8506 ;KPACK - 5/16/2018
8510 .MACRO MULTIPLY
8520 LDA %1 ;MULTIPLICAND 1
8530 STA W5
8540 LDA %1+1
8550 STA W5+1
8560 LDA %2 ;MULTIPLICAND 2
8570 STA W6
8580 LDA %2+1
8590 STA W6+1
8600 LDA #MULTIPLY
8610 JSR DIAMOND
8620 .ENDM
8630 ;

The test program:
10 ;THIS PROGRAM IS TO TEST CHANGES
20 ;TO THE DIAMOND MULTIPLY MACRO
30 ;
40 ;MACRO NOW LOAD NUMBERS AT ADDR
50 ; -NOT NUMBERS
60 ;DOUBLE BYTE MATH ON TWO NUMBERS.
70 ;RESULTS PRINTED IN HEX.
80 ;
90 ;
95 .OPT NO LIST
96 .OPT OBJ
0100 *= $2200
0110 .INCLUDE #D2:DMACRO01.M65
0120 ;
0130 NUM01 .WORD 1211
0140 NUM02 .WORD 37
0150 MULRST .WORD 0 ; MUL RESULTS
0180 DVAR .WORD 0
0190 HEXSTG .BYTE "$ ",255
0200 HEXCHR .BYTE $30,$31,$32,$33,$34
0210 .BYTE $35,$36,$37,$38,$39
0220 .BYTE $41,$42,$43,$44,$45
0230 .BYTE $46
0240 ;
0250 START INIT 0
0260 MULTIPLY NUM01,NUM02
0270 LDA W7 ;STORE RESULTS
0280 STA MULRST
0290 LDA W7+1
0300 STA MULRST+1
0400 LDA NUM01 ;LOAD NUM TO CONVERT
0410 STA DVAR
0420 LDA NUM01+1
0430 STA DVAR+1
0440 JSR DB2HEX
0450 SYSDRAW HEXSTG,10,20,0
0460 LDA NUM02 ;LOAD NUM TO CONVERT
0470 STA DVAR
0480 LDA NUM02+1
0490 STA DVAR+1
0500 JSR DB2HEX
0510 SYSDRAW HEXSTG,10,30,0
0520 LDA MULRST ;LOAD NUM TO CONVERT
0530 STA DVAR
0540 LDA MULRST+1
0550 STA DVAR+1
0560 JSR DB2HEX
0570 SYSDRAW HEXSTG,10,40,0
0691 LOOP EVENT
0692 LDA EVENTTYPE
0693 BEQ LOOP
0694 EXIT ;DIAMOND
0695 EXECDESKTOP
0696 ;
0700 ;
0710 ;CONVERT HEX TO STRING
0720 DB2HEX LDA DVAR
0730 AND #$0F
0740 TAX
0750 LDA HEXCHR,X
0760 STA HEXSTG+4
0770 LSR DVAR
0780 LSR DVAR
0790 LSR DVAR
0800 LSR DVAR
0810 LDX DVAR
0820 LDA HEXCHR,X
0830 STA HEXSTG+3
0840 LDA DVAR+1
0850 AND #$0F
0860 TAX
0870 LDA HEXCHR,X
0880 STA HEXSTG+2
0890 LSR DVAR+1
0900 LSR DVAR+1
0910 LSR DVAR+1
0930 LSR DVAR+1
0935 LDX DVAR+1
0940 LDA HEXCHR,X
0950 STA HEXSTG+1
0960 RTS
0970 ;
0980 *= $02E0
0990 .WORD START

My test results are as follows:

TEST# NUM01 * NUM02 = Result Correct
1 $0B44 $0025 $2F07 $AF07
2 $FFF6 $0025 $7E8E
3 $1FFF $0004 $7FFC
4 $1FFF $0005 $9FFB
5 $0004 $1FFF $7FFC
6 $0005 $1FFF $9FFB
7 $00FF $00FF $FE01
8 $FFFF $0001 $7FFFF $FFFF
9 $0001 $04BB $04BB
10 $0001 $FFFF ---- CRASH

Every time I look at this list, I see numbers I should have tried. I just got tired of trying. There seems to be a problem when the result is above $7FFF but not always(#4). Switching the numbers can have unexpected results(#8 and #10). Zero should have been tested.

The only recommendations I will make at this time is to know the limits of the numbers you will be using in your program and test them using the macro call.

Now, I'm hoping I will remember that I wrote those words of wisdom the next time I have the need to call Diamond's MULTIPLY macro.