Here’s what the Cromix boot sector (for Cromix 11.27) looks like and why …<\/p>\n
<\/code> It would be elegant if just changing those few bytes changed the behaviour of the boot sector so you could use the same program on all boot sectors; but it isn’t quite that good. Just looking above, you can see much of the sector is zeros – so there isn’t a lot of bytes for “smarts”. Despite that, it does have some tied to the “DS” and “DD” bits. Where it fails to adapt is if you copy (or WRTSYS) between disk sizes (5->8 or 8->5).<\/p>\n Here’s what the program does: <\/code> Cromix is loading into bank 0 of memory. It has all of bank 0 to itself, so it can load anywhere it likes. In CP\/M and CDOS, 100H is reserved for user programs.<\/p>\n I don’t have (and haven’t created) a disassembly of RDOS0312; but there is one for RDOS0252. It contains: <\/code> For my purposes, to boot without RDOS, I patch the boot.hex program I use with z80sim to “NOP-out” the “JR C, 008AH” line. This means it boots from a single density, 8″ track 0 in drivea.dsk. The image on the disk is unchanged so if you boot a disk within cromix – the SCF override still works.<\/p>\n","protected":false},"excerpt":{"rendered":" Here’s what the Cromix boot sector (for Cromix 11.27) looks like and why …<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[16,59],"tags":[],"_links":{"self":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1782"}],"collection":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/comments?post=1782"}],"version-history":[{"count":3,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1782\/revisions"}],"predecessor-version":[{"id":1785,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1782\/revisions\/1785"}],"wp:attachment":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/media?parent=1782"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/categories?post=1782"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/tags?post=1782"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}<\/p>\n\r\n Display of File BOOT1.BIN\r\n\r\n000000: 3E 01 D3 40 38 04 97 08 16 31 21 00 01 01 0F 02 >..@8....1!.....\r\n000010: 3E 7F D3 04 7A D3 34 79 D3 30 97 3D 20 FD DB 34 >...z.4y.0.= ..4\r\n000020: 1F 30 FB DB 30 E6 98 20 D7 78 D3 32 7A F6 80 D3 .0..0.. .x.2z...\r\n000030: 34 0E 33 3E 9C D3 30 DB 34 1F 38 05 ED A2 C3 B7 4.3>..0.4.8.....\r\n000040: 00 DB 30 CB 67 28 B9 C3 00 01 00 00 00 00 00 00 ..0.g(..........\r\n000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................\r\n000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................\r\n000070: 00 00 00 00 00 00 00 00 43 4C 44 53 44 44 E5 E5 ........CLDSDD..\r\n\r\n<\/pre>\n
\nYou can see the disk type label at bytes 120-127 (0x78..0x7F). It is a Cromix (“C”), Large (“L” aka 8 inch), double-sided (“DS”), double-density (“DD”) disk. If this isn’t present and correct then the boot sequence won’t work and parts of the OS and\/or utilities will get things wrong. That doesn’t apply to early versions of CDOS or to CP\/M but it does apply to CDOS 2.58 and Cromix.<\/p>\n
\n<\/p>\n\r\n ORG 0080H\r\n0080 3e 01 ld a,0x01 ; disable RDOS\r\n0082 d3 40 out (0x40),a\r\n\r\n0084 38 04 jr c,0x008a ; if CY use supplied\r\n0086 97 sub a ; default disk=0\r\n0087 08 ex af,af' ; save for later\r\n0088 16 31 ld d,0x31 ; default cmd=motor+8\"+A \r\n\r\n008a 21 00 01 ld hl,0x0100 ; we're going to load to 100H\r\n008d 01 0f 02 ld bc,0x020f ; B=sector 2, C=restore\r\n0090 3e 7f ld a,0x7f ; select side 0\r\n0092 d3 04 out (0x04),a\r\n\r\n0094 7a ld a,d ; 31=motor+8\"+A (or supplied)\r\n0095 d3 34 out (0x34),a\r\n0097 79 ld a,c ; cmd. 0F=restore+verify+slow\r\n0098 d3 30 out (0x30),a\r\n\r\n009a 97 sub a ; wait 256\r\n009b 3d dec a\r\n009c 20 fd jr nz,0x009b\r\n\r\n009e db 34 in a,(0x34) ; wait for end of job, EOJ\r\n00a0 1f rra\r\n00a1 30 fb jr nc,0x009e\r\n\r\n00a3 db 30 in a,(0x30) ; check result\r\n00a5 e6 98 and 0x98 ; notRdy+notFound+busy\r\n00a7 20 d7 jr nz,0x0080 ; problem. try again. CY=0 btw\r\n\r\n00a9 78 ld a,b ; set sector (2 at start)\r\n00aa d3 32 out (0x32),a\r\n\r\n00ac 7a ld a,d ; motor+8\"+A (or supplied)\r\n00ad f6 80 or 0x80 ; turn on autowait\r\n00af d3 34 out (0x34),a\r\n\r\n00b1 0e 33 ld c,0x33 ; 33H is the disk data port\r\n00b3 3e 9c ld a,0x9c ; read records, head load\r\n00b5 d3 30 out (0x30),a ; send the cmd\r\n\r\n00b7 db 34 in a,(0x34) ; check for EOJ\r\n00b9 1f rra\r\n00ba 38 05 jr c,0x00c1 ; read all sectors to end of track\r\n00bc ed a2 ini ; read bytes to HL...\r\n00be c3 b7 00 jp 0x00b7 ; loop until EOJ\r\n\r\n00c1 db 30 in a,(0x30) ; check result\r\n00c3 cb 67 bit 4,a ; the \"not found\" bit\r\n00c5 28 b9 jr z,0x0080 ; error. try again\r\n00c7 c3 00 01 jp 0x0100 ; all ok. run what we loaded.\r\n00ca 00 nop\r\n00cb 00 nop\r\n...\r\n00f6 00 nop\r\n00f7 00 nop\r\n00f8 43 4C 44 DB 'CLDSDD',0xe5,0xe5\r\n00fb 53 44 44\r\n00fe e5 e5\r\n<\/pre>\n
\nThere’s a number of interesting things here:
\n– it only loads the rest of track 0 (CP\/M and CDOS use tracks 0 and 1 for 8″ and 0-2 for 5″)
\n– it loads to 100H; instead of somewhere high
\n– despite my statement earlier, it doesn’t care about “CLDSDD”. (Other things do)
\n– it depends of the CY flag at startup (and A and D)<\/p>\n
\n<\/p>\n\r\nC0FA:\tCALL\tckesc\t; look for escape\r\n\tCALL\tsetsel\r\n\tLD\tD,A\r\n\tLD\tA,(77H)\r\n\tEX\tAF,AF'\r\n\tCALL\tptfol\r\n\tDB CR,'Standby',CR+msbon\r\n\tSCF\r\n\tJP\t80H\r\n<\/pre>\n
\nYou can see the value in D getting created (setsel), a value being saved in A’, and SCF just before running the boot sector. “ptfol” prints the following bytes (until one with the most significant bit set, msbon=0x80). CRs get printed as CR, LF. “setsel” builds a bit combination based on whether you’re booting from an 8″ or 5″ drive, which drive it is, and whether it is single density or double density. All those smarts are in RDOS (probably just based on what you typed in as the RDOS command\/s or defaults).<\/p>\n