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peasant: add random number generator

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This commit is contained in:
Vince Weaver 2021-10-31 18:46:23 -04:00
parent b49e5f00e8
commit a7f0280aae
4 changed files with 124 additions and 1 deletions
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4
games/peasant/Makefile
View File

@ -74,7 +74,8 @@ qload.o: qload.s qboot.inc \
clear_bottom.s \
hgr_hgr2.s \
gr_offsets.s \
qkumba_popwr.s
qkumba_popwr.s \
random16.s
ca65 -o qload.o qload.s -l qload.lst
###
@ -136,6 +137,7 @@ qload.inc: generate_common QLOAD
./generate_common -a 0xb00 -s location_names_l qload.lst >> qload.inc
./generate_common -a 0xb00 -s location_names_h qload.lst >> qload.inc
./generate_common -a 0xb00 -s wait_until_keypress qload.lst >> qload.inc
./generate_common -a 0xb00 -s random16 qload.lst >> qload.inc
echo "hposn_high = \$$BA00" >> qload.inc
echo "hposn_low = \$$BB00" >> qload.inc
echo "driveoff = \$$A22" >> qload.inc

1
games/peasant/keyboard.s
View File

@ -21,6 +21,7 @@ check_keyboard:
rts
key_was_pressed:
inc SEEDL
and #$5f ; strip off high bit and make uppercase

1
games/peasant/qload.s
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@ -257,6 +257,7 @@ length_array:
.include "gr_offsets.s"
.include "loadsave_menu.s"
.include "wait_keypress.s"
.include "random16.s"
peasant_text:
.byte 25,2,"Peasant's Quest",0

119
games/peasant/random16.s Normal file
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@ -0,0 +1,119 @@
; 16-bit 6502 Random Number Generator (cycle-invariant version)
; Linear feedback shift register PRNG by White Flame
; http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
; The Apple II KEYIN routine increments SEEDL:SEEDH
; while waiting for keypress
;SEEDL = $4E
;SEEDH = $4F
XOR_MAGIC = $7657 ; "vW"
;=============================
; random16
;=============================
; takes:
; not 0, cs = 6(r16)+12(lnz)+5(nop)+ 19(deo) = 42
; not 0, cc = 6(r16)+14(lnz)+2(nop)+ 20(neo) = 42
; $0000 = 6(r16)+ 6(loz)+11nops+ 19(deo) = 42
; $8000 = 6(r16)+ 6(loz)+ 4(ceo) + 6nops+ 20(neo) = 42
; $XX00 cc = 6(r16)+ 6(loz)+4(ceo)+2(cep) +4nops+ 20(neo) = 42
; $XX00 cs = 6(r16)+ 6(loz)+4(ceo)+4(cep) +3nops+ 19(deo) = 42*
random16:
lda SEEDL ; 3
beq low_zero ; $0000 and $8000 are special values ; 3
;==========
; 6
lownz:
; -1
asl SEEDL ; Do a normal shift ; 5
lda SEEDH ; 3
rol ; 2
bcs five_cycle_do_eor ; 3
;===========
; 12
bcc two_cycle_no_eor ; 3
;==========
; 12+3-1 = 14
;===================================================================
eleven_cycle_do_eor:
nop ; 2
nop ; 2
nop ; 2
five_cycle_do_eor:
nop ; 2
three_cycle_do_eor:
sta SEEDH ; nop ; 3
do_eor:
; high byte is in A
eor #>XOR_MAGIC ; 2
sta SEEDH ; 3
lda SEEDL ; 3
eor #<XOR_MAGIC ; 2
sta SEEDL ; 3
eor_rts:
rts ; 6
;===========
; 19
;=========================================================================
six_cycles_no_eor:
nop ; 2
four_cycle_no_eor:
nop ; 2
two_cycle_no_eor:
nop ; 2
no_eor:
nop ; 2
nop ; 2
nop ; 2
nop ; 2
sta SEEDH ; 3
jmp eor_rts ; 3+6
;===========
; 20
;======================================================================
;======================================================================
low_zero:
lda SEEDH ; 3
beq eleven_cycle_do_eor ; High byte is also zero ; 3
; so apply the EOR
;============
; 6
ceo:
; -1
; wasn't zero, check for $8000
asl ; 2
beq six_cycles_no_eor ; if $00 is left after the shift; 3
; then it was $80
;===========
; 4
; else, do the EOR based on the carry
cep:
; -1
bcc four_cycle_no_eor ; 3
;============
; 2
bcs three_cycle_do_eor ; 2+3-1 = 4