add a test program for multiplication

src/MATHTST.ASM

@@ -0,0 +1,58 @@
+; Test of different math functions
+; Uses intel 8080 mnemonics
+            ORG 100H         ; CP/M program start here
+            JMP START        ; jump to the program entry point
+; Variable storage space
+MsgStr: DB 'Correct.',13,10,0
+Stack:      DW 0             ; place to store the old stack
+SBOT:       DS 32            ; reserve 32 bytes of stack
+STOP:       EQU $-1          ; points to the top of the stack
+BDOS        EQU 5            ; address for BDOS calls in CP/M
+; Start of the code segment
+; PRINTF: Prints a string on the screen using a BDOS function. 
+; The address of the string is expected to be in register HL.
+PRINTF:
+PRLOOP:    MOV A, M         ; read next char
+		   ORA A            ; check, if string has ended
+		   JZ  PREND        ; if char == 0, then leave
+		   MOV E, A         ; load the char into E
+		   MVI C, 2         ; call BDOS function id 2
+		   PUSH H           ; save the HL register
+		   CALL BDOS        ; call the BDOS function
+		   POP H            ; restore the HL register
+		   INX H            ; point to the next char
+		   JMP PRLOOP       ; do the next char
+PREND:     RET              ; return from subroutine
+; MUL: Multiplicates two 8 bit numbers by using right shifts
+;      The multiplicator must be in C and the multiplicand in D
+MUL:       MVI B, 0         ; initialize the most significant byte
+                            ; of the result
+           MVI E, 9         ; set the bit counter (is bits + 1)
+MULT0:     MOV A, C         ; rotate the least significant bit of 
+           RAR              ; multiplier to carry and shift
+           MOV C, A         ; low order byte of result
+           DCR E            ; decrease the bit counter
+           JZ MULEND        ; exit, if all bits are done
+           MOV A, B         ; load the most significant bit
+           JNC MULT1        ; continue with rotation, when carry not set
+           ADD D            ; Add multiplicant to high-order byte of result, 
+                            ; if the bit was 1
+MULT1:     RAR              ; shift hight-order byte of result
+           MOV B, A         ; load the high-order byte back to B
+           JMP MULT0        ; jump to beginning of loop
+MULEND:    RET              ; return from subroutine
+START:     LXI H, 0         ; blank out HL
+		   DAD SP           ; HL = SP
+		   SHLD Stack       ; save the original stack
+		   LXI H, STOP      ; set HL to the new stack top
+		   SPHL             ; set SP to point to our new stack
+           LXI D, 0BH       ; loads a value into the multiplicant
+           LXI C, 09H       ; loads multiplier to C
+           CALL MUL         ; multiply the two values
+           LXI A, 63H       ; load the expected result into the accumulator
+           CMP C            ; compare the result.
+           JNZ PROGEND      ; if the result is not met, fail.
+           LXI H, MsgStr    ; load the message
+           CALL PRINTF      ; print the results
+PROGEND:   RET              
+           END