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⌂ ∞ ▽ Elite on the 6502

Universe: TT24

[NES version, Bank 6]

       Name: TT24                                                    [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Calculate system data from the system seeds
  Deep dive: Generating system data
             Galaxy and system seeds

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT24_b6 calls TT24




 Calculate system data from the seeds in QQ15 and store them in the relevant
 locations. Specifically, this routine calculates the following from the three
 16-bit seeds in QQ15 (using only s0_hi, s1_hi and s1_lo):

   QQ3 = economy (0-7)
   QQ4 = government (0-7)
   QQ5 = technology level (0-14)
   QQ6 = population * 10 (1-71)
   QQ7 = productivity (96-62480)

 The ranges of the various values are shown in brackets. Note that the radius
 and type of inhabitant are calculated on-the-fly in the TT25 routine when
 the system data gets displayed, so they aren't calculated here.



.TT24

 LDA QQ15+1             ; Fetch s0_hi and extract bits 0-2 to determine the
 AND #%00000111         ; system's economy, and store in QQ3
 STA QQ3

 LDA QQ15+2             ; Fetch s1_lo and extract bits 3-5 to determine the
 LSR A                  ; system's government, and store in QQ4
 LSR A
 LSR A
 AND #%00000111
 STA QQ4

 LSR A                  ; If government isn't anarchy or feudal, skip to TT77,
 BNE TT77               ; as we need to fix the economy of anarchy and feudal
                        ; systems so they can't be rich

 LDA QQ3                ; Set bit 1 of the economy in QQ3 to fix the economy
 ORA #%00000010         ; for anarchy and feudal governments
 STA QQ3

.TT77

 LDA QQ3                ; Now to work out the tech level, which we do like this:
 EOR #%00000111         ;
 CLC                    ;   flipped_economy + (s1_hi AND %11) + (government / 2)
 STA QQ5                ;
                        ; or, in terms of memory locations:
                        ;
                        ;   QQ5 = (QQ3 EOR %111) + (QQ15+3 AND %11) + (QQ4 / 2)
                        ;
                        ; We start by setting QQ5 = QQ3 EOR %111

 LDA QQ15+3             ; We then take the first 2 bits of s1_hi (QQ15+3) and
 AND #%00000011         ; add it into QQ5
 ADC QQ5
 STA QQ5

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA QQ4                ; And finally we add QQ4 / 2 and store the result in
 LSR A                  ; QQ5, using LSR then ADC to divide by 2, which rounds
 ADC QQ5                ; up the result for odd-numbered government types
 STA QQ5

 ASL A                  ; Now to work out the population, like so:
 ASL A                  ;
 ADC QQ3                ;   (tech level * 4) + economy + government + 1
 ADC QQ4                ;
 ADC #1                 ; or, in terms of memory locations:
 STA QQ6                ;
                        ;   QQ6 = (QQ5 * 4) + QQ3 + QQ4 + 1

 LDA QQ3                ; Finally, we work out productivity, like this:
 EOR #%00000111         ;
 ADC #3                 ;  (flipped_economy + 3) * (government + 4)
 STA P                  ;                        * population
 LDA QQ4                ;                        * 8
 ADC #4                 ;
 STA Q                  ; or, in terms of memory locations:
 JSR MULTU              ;
                        ;   QQ7 = (QQ3 EOR %111 + 3) * (QQ4 + 4) * QQ6 * 8
                        ;
                        ; We do the first step by setting P to the first
                        ; expression in brackets and Q to the second, and
                        ; calling MULTU, so now (A P) = P * Q. The highest this
                        ; can be is 10 * 11 (as the maximum values of economy
                        ; and government are 7), so the high byte of the result
                        ; will always be 0, so we actually have:
                        ;
                        ;   P = P * Q
                        ;     = (flipped_economy + 3) * (government + 4)

 LDA QQ6                ; We now take the result in P and multiply by the
 STA Q                  ; population to get the productivity, by setting Q to
 JSR MULTU              ; the population from QQ6 and calling MULTU again, so
                        ; now we have:
                        ;
                        ;   (A P) = P * population

 ASL P                  ; Next we multiply the result by 8, as a 16-bit number,
 ROL A                  ; so we shift both bytes to the left three times, using
 ASL P                  ; the C flag to carry bits from bit 7 of the low byte
 ROL A                  ; into bit 0 of the high byte
 ASL P
 ROL A

 STA QQ7+1              ; Finally, we store the productivity in two bytes, with
 LDA P                  ; the low byte in QQ7 and the high byte in QQ7+1
 STA QQ7

 RTS                    ; Return from the subroutine