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Lecture37W_F

Lecture37W_F - Multiprecision Arithmetic Arithmetic on...

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Page 1 of 2 Multiprecision Arithmetic Arithmetic on numbers larger than data format BCD numbers larger than 2 digits (1 byte) Standard binary numbers larger than register (16 bits = 2 bytes) BCD As observed in Lab Exercise 6, BCD instructions use carry (“C”) bit as well as half (“ H ”) bit. C bit of CCR - Carry for addition - Borrow for subtraction Example: Subroutine to add 2 BCD strings of 16 BCD digits: String1 = String1 + String2 BCD_BYTES: EQU 8 ; # bytes for 16 BCD digits AddBCD: ldab #BCD_BYTES ;Loop counter ldx #(String1+BCD_BYTES) ;String1 ptr ldy #(String2+BCD_BYTES) ;String2 ptr clc ;clear C bit if CCR Loop: ;repeat { ldaa 1,-x ; A = byte of String1 adca 1,-y ; + byte of String2 daa ; Adjust A for BCD add staa 0,x ; Store in String1 dbne b,Loop ;} until (--B == 0)

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