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Elite on the BBC Micro

Universe: GVL [Disc version, Flight]

Name: GVL [Show more] Type: Subroutine Category: Universe Summary: Calculate the availability of market items Deep dive: Market item prices and availability Galaxy and system seeds
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

Calculate the availability for each market item and store it in AVL. This is called on arrival in a new system. Other entry points: hyR Contains an RTS
.GVL JSR DORND \ Set A and X to random numbers STA QQ26 \ Set QQ26 to the random byte that's used in the market \ calculations LDX #0 \ We are now going to loop through the market item STX XX4 \ availability table in AVL, so set a counter in XX4 \ (and X) for the market item number, starting with 0 .hy9 LDA QQ23+1,X \ Fetch byte #1 from the market prices table (units and STA QQ19+1 \ economic_factor) for item number X and store it in \ QQ19+1 JSR var \ Call var to set QQ19+3 = economy * |economic_factor| \ (and set the availability of alien items to 0) LDA QQ23+3,X \ Fetch byte #3 from the market prices table (mask) and AND QQ26 \ AND with the random number for this system visit \ to give: \ \ A = random AND mask CLC \ Add byte #2 from the market prices table ADC QQ23+2,X \ (base_quantity) so we now have: \ \ A = base_quantity + (random AND mask) LDY QQ19+1 \ Fetch the byte #1 that we stored above and jump to BMI TT157 \ TT157 if it is negative (i.e. if the economic_factor \ is negative) SEC \ Set A = A - QQ19+3 SBC QQ19+3 \ \ = base_quantity + (random AND mask) \ - (economy * |economic_factor|) \ \ which is the result we want, as the economic_factor \ is positive JMP TT158 \ Jump to TT158 to skip TT157 .TT157 CLC \ Set A = A + QQ19+3 ADC QQ19+3 \ \ = base_quantity + (random AND mask) \ + (economy * |economic_factor|) \ \ which is the result we want, as the economic_factor \ is negative .TT158 BPL TT159 \ If A < 0, then set A = 0, so we don't have negative LDA #0 \ availability .TT159 LDY XX4 \ Fetch the counter (the market item number) into Y AND #%00111111 \ Take bits 0-5 of A, i.e. A mod 64, and store this as STA AVL,Y \ this item's availability in the Y=th byte of AVL, so \ each item has a maximum availability of 63t INY \ Increment the counter into XX44, Y and A TYA STA XX4 ASL A \ Set X = counter * 4, so that X points to the next ASL A \ item's entry in the four-byte market prices table, TAX \ ready for the next loop CMP #63 \ If A < 63, jump back up to hy9 to set the availability BCC hy9 \ for the next market item .hyR RTS \ Return from the subroutine