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Elite on the BBC Micro

Maths (Arithmetic): LL62 [Elite-A, Docked]

Name: LL62 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate 128 - (U R)
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * LL9 (Part 8 of 12) calls LL62

Calculate the following for a positive sign-magnitude number (U R): 128 - (U R) and then store the result, low byte then high byte, on the end of the heap at XX3, where X points to the first free byte on the heap. Return by jumping down to LL66. Returns: X X is incremented by 1
.LL62 LDA #128 \ Calculate 128 - (U R), starting with the low bytes SEC SBC R STA XX3,X \ Store the low byte of the result in the X-th byte of \ the heap at XX3 INX \ Increment the heap pointer in X to point to the next \ byte LDA #0 \ And then subtract the high bytes SBC U STA XX3,X \ Store the low byte of the result in the X-th byte of \ the heap at XX3 JMP LL66 \ Jump down to LL66