The first piece of OS-9 Level 1 loads from the boot track (track 34, sectors 1-18) into memory starting at $2600.
A two-byte signature is followed by the 2 byte execution start address.
The RELocate code copies the boot track from $2600 up into $F000 and transfers control there into the Kernel (part 1) module after RELocation.
ORG $2600
BOOTSIG FCC "OS" ;2600: 4F 53 'OS' DOS BOOTTRACK OS SIGNATURE
EXECADDR FDB REL ;2602: 26 0C '&.' EXECUTION START ADDRESS
OS9_BOOT FCC "OSy`BOOT" ;2604: 4F 53 79 60 42 4F 4F 54 'OSy`BOOT' OS-9 BOOT SCREEN STRING
; START OF REL "MODULE"
REL CLR PIA0BC ;260C: 7F FF 03 '...' DISABLE VSYNC
STA SAMTYSET ;260F: B7 FF DF '...' SAM MAP TYPE=1 "ALL RAM"
LDB #$06 ;2612: C6 06 '..' SET COUNTDOWN TO CLEAR 7 BITS (B=6)
LDX #SAMF0CLR ;2614: 8E FF C6 '...' START WITH SAM DISPLAY OFFSET F0 BIT
REL10 STA ,X++ ;2617: A7 81 '..' CLEAR THE DISPLAY OFFSET BIT THEN X=X+2
DECB ;2619: 5A 'Z' COUNT DOWN 1
BNE REL10 ;261A: 26 FB '&.' DID WE CLEAR OUR 7TH BIT (B=0)
STA $01,X ;261C: A7 01 '..' YES, SET DISPLAY OFFSET SCREEN AT $8000
; CLEAR SCREEN WITH SPACES
LDX #M8000 ;261E: 8E 80 00 '...'
LDY #M0200 ;2621: 10 8E 02 00 '....'
LDA #$60 ;2625: 86 60 '.`'
REL20 STA ,X+ ;2627: A7 80 '..'
DEY ;2629: 31 3F '1?'
BNE REL20 ;262B: 26 FA '&.'
; PUT BANNER ONTO SCREEN
LDX #M810C ;262D: 8E 81 0C '...'
LEAY OS9_BOOT,PCR ;2630: 31 8C D1 '1..'
LDB #$08 ;2633: C6 08 '..'
REL30 LDA ,Y+ ;2635: A6 A0 '..'
STA ,X+ ;2637: A7 80 '..'
DECB ;2639: 5A 'Z'
BNE REL30 ;263A: 26 F9 '&.'
; FIXME WHAT IS THIS
TST M0072 ;263C: 0D 72 '.r' FIXME TEST ALREADY RELOCATED FLAG?
BEQ REL50 ;263E: 27 13 ''.'
; START RELOCATING
LEAU >BOOTSIG,PCR ;2640: 33 8D FF BC '3...'
LDX #M0F00 ;2644: 8E 0F 00 '...'
LDY #MF000 ;2647: 10 8E F0 00 '....'
REL40 LDA ,U+ ;264B: A6 C0 '..'
STA ,Y+ ;264D: A7 A0 '..'
DEX ;264F: 30 1F '0.'
BNE REL40 ;2651: 26 F8 '&.'
; RELOCATION COMPLETE, PREPARE FOR OS9 MODULE COLD START
REL50 JMP ZF056 ;2653: 7E F0 56 '~.V' JUMP TO RELOCATED MODULES
LEAX <M01SYNC,PCR ;2656: 30 8C 04 '0..' LOCATE OS9 MODULE START
LDD $09,X ;2659: EC 09 '..' LOCATE OS9 EXEC ADDRESS
JMP D,X ;265B: 6E 8B 'n.' JUMP INTO OS9 MODULE
▲ OS-9