Universe: PDESC

       Name: PDESC                                                   [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Print the system's extended description or a mission 1 directive
  Deep dive: Extended system descriptions
             Extended text tokens
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT25 calls PDESC


 This prints a specific system's extended description. This is called the "pink
 volcanoes string" in a comment in the original source, and the "goat soup"
 recipe by Ian Bell on his website (where he also refers to the species string
 as the "pink felines" string).

 For some special systems, when you are docked at them, the procedurally
 generated extended description is overridden and a text token from the RUTOK
 table is shown instead. If mission 1 is in progress, then a number of systems
 along the route of that mission's story will show custom mission-related
 directives in place of that system's normal "goat soup" phrase.


 Arguments:

   ZZ                   The system number (0-255)


.PDESC

 LDA QQ8                \ If either byte in QQ18(1 0) is non-zero, meaning that
 ORA QQ8+1              \ the distance from the current system to the selected
 BNE PD1                \ is non-zero, jump to PD1 to show the standard "goat
                        \ soup" description

                        \ If we get here, then the current system is the same as
                        \ the selected system and we are docked, so now to check
                        \ whether there is a special override token for this
                        \ system

 LDY #NRU%              \ Set Y as a loop counter as we work our way through the
                        \ system numbers in RUPLA, starting at NRU% (which is
                        \ the number of entries in RUPLA, 26) and working our
                        \ way down to 1

.PDL1

 LDA RUPLA-1,Y          \ Fetch the Y-th byte from RUPLA-1 into A (we use
                        \ RUPLA-1 because Y is looping from 26 to 1)

 CMP ZZ                 \ If A doesn't match the system whose description we
 BNE PD2                \ are printing (in ZZ), jump to PD2 to keep looping
                        \ through the system numbers in RUPLA

                        \ If we get here we have found a match for this system
                        \ number in RUPLA

 LDA RUGAL-1,Y          \ Fetch the Y-th byte from RUGAL-1 into A

 AND #%01111111         \ Extract bits 0-6 of A

 CMP GCNT               \ If the result does not equal the current galaxy
 BNE PD2                \ number, jump to PD2 to keep looping through the system
                        \ numbers in RUPLA

 LDA RUGAL-1,Y          \ Fetch the Y-th byte from RUGAL-1 into A, once again

 BMI PD3                \ If bit 7 is set, jump to PD3 to print the extended
                        \ token in A from the second table in RUTOK

 LDA TP                 \ Fetch bit 0 of TP into the C flag, and skip to PD1 if
 LSR A                  \ it is clear (i.e. if mission 1 is not in progress) to
 BCC PD1                \ print the "goat soup" extended description

                        \ If we get here then mission 1 is in progress, so we
                        \ print out the corresponding token from RUTOK

 JSR MT14               \ Call MT14 to switch to justified text

 LDA #1                 \ Set A = 1 so that extended token 1 (an empty string)
                        \ gets printed below instead of token 176, followed by
                        \ the Y-th token in RUTOK

 EQUB &2C               \ Skip the next instruction by turning it into
                        \ &2C &A9 &B0, or BIT &B0A9, which does nothing apart
                        \ from affect the flags

.PD3

 LDA #176               \ Print extended token 176 ("{lower case}{justify}
 JSR DETOK2             \ {single cap}")

 TYA                    \ Print the extended token in Y from the second table
 JSR DETOK3             \ in RUTOK

 LDA #177               \ Set A = 177 so when we jump to PD4 in the next
                        \ instruction, we print token 177 (".{cr}{left align}")

 BNE PD4                \ Jump to PD4 to print the extended token in A and
                        \ return from the subroutine using a tail call

.PD2

 DEY                    \ Decrement the byte counter in Y

 BNE PDL1               \ Loop back to check the next byte in RUPLA until we
                        \ either find a match for the system in ZZ, or we fall
                        \ through into the "goat soup" extended description
                        \ routine

.PD1

                        \ We now print the "goat soup" extended description

 LDX #3                 \ We now want to seed the random number generator with
                        \ the s1 and s2 16-bit seeds from the current system, so
                        \ we get the same extended description for each system
                        \ every time we call PDESC, so set a counter in X for
                        \ copying 4 bytes

{
.PDL1                   \ This label is a duplicate of the label above (which is
                        \ why we need to surround it with braces, as BeebAsm
                        \ doesn't allow us to redefine labels, unlike BBC BASIC)

 LDA QQ15+2,X           \ Copy QQ15+2 to QQ15+5 (s1 and s2) to RAND to RAND+3
 STA RAND,X

 DEX                    \ Decrement the loop counter

 BPL PDL1               \ Loop back to PDL1 until we have copied all

 LDA #5                 \ Set A = 5, so we print extended token 5 in the next
                        \ instruction ("{lower case}{justify}{single cap}[86-90]
                        \ IS [140-144].{cr}{left align}"
}

.PD4

 JMP DETOK              \ Print the extended token given in A, and return from
                        \ the subroutine using a tail call