The first thing I found is that only part of high memory is initialised when you first start the system. A lot more is loaded into high memory when you issue a USER command.
To investigate further, I used MEMU to emulate an SDX, issued the command USER DIR "A:", and then dumped the contents of memory to a file (<F9+d> for Andy's MEMU, or <F10+d> for my fork). I am now attempting to disassemble the resulting memory dump. Some of the address labels can be obtained from file SDXHIGH.INC from Andy's ROM source here.
I have only just started, but have already come across a few puzzles:
- The start of the equivalent of BDOS is at D706. The BDOS function code (C register) is eventually tested for validity at D733. However the test is for values less than #29 (hex=41 dec). However my documentation of BDOS functions for CP/M 2.2 only goes up to #24. So what are #25 - #28?
- The value of C is then used to select an address from a jump table starting at D747. However, looking at some of the addresses in this table:
- #02. Console output => Address D890.
- #0A. Read console buffer => Address D8E1.
- These addresses fall in the range occupied by the file control blocks CHNL1 to CHNL5 (D840 to D911). In the memory dump this range is mostly zero filled, but in general will change as files are opened and closed. So these two BDOS functions clearly cannot work.
- Similarly, the routine starting at D823 contains jumps to D845 (from D827) and D842 (from d835). So again this routine cannot work.
![Confused :?](./images/smilies/icon_e_confused.gif)
Code: Select all
LSTHIGH: DEFB #4B, #16, #00, #00, #1F, #69 ; D700 D700 4B 16 00 00 1F 69 Start of BDOS (JP JP59K)
JP59K: JP LD711 ; D706 D706 C3 11 D7
LD709: SBC A,C ; D709 D709 99
RST L0010 ; D70A D70A D7
DEFB #A5 ; D70B D70B A5 Send 5 bytes to screen.
DEFS "\D7\AB\D7\B1\D7" ; D70C D70C D7 AB D7 B1 D7
LD711: EX DE,HL ; D711 D711 EB
LD (XDA43),HL ; D712 D712 22 43 DA
EX DE,HL ; D715 D715 EB
LD A,E ; D716 D716 7B
LD (XE4D6),A ; D717 D717 32 D6 E4
LD HL,#0000 ; D71A D71A 21 00 00
LD (XDA45),HL ; D71D D71D 22 45 DA
ADD HL,SP ; D720 D720 39
LD (XDA0F),HL ; D721 D721 22 0F DA
LD SP,XDA00 ; D724 D724 31 00 DA
XOR A ; D727 D727 AF
LD (XE4E0),A ; D728 D728 32 E0 E4
LD (XE4DE),A ; D72B D72B 32 DE E4
LD HL,LE474 ; D72E D72E 21 74 E4
PUSH HL ; D731 D731 E5
LD A,C ; D732 D732 79
CP #29 ; D733 D733 FE 29
RET NC ; D735 D735 D0
LD C,E ; D736 D736 4B
LD HL,XD747 ; D737 D737 21 47 D7
LD E,A ; D73A D73A 5F
LD D,#00 ; D73B D73B 16 00
ADD HL,DE ; D73D D73D 19
ADD HL,DE ; D73E D73E 19
LD E,(HL) ; D73F D73F 5E
INC HL ; D740 D740 23
LD D,(HL) ; D741 D741 56
LD HL,(XDA43) ; D742 D742 2A 43 DA
EX DE,HL ; D745 D745 EB
JP (HL) ; D746 D746 E9
XD747: DEFW LE503, LD9C8, LD890, LD9CE, LE512, LE50F, LD9D4, LD9ED ; D747 D747 03 E5 C8 D9 90 D8 CE D9 12 E5 0F E5 D4 D9 ED D9
DEFW LD9F3, LD9F8, LD8E1, LD9FE, LE37E, LE383, LE345, LE39C ; D757 D757 F3 D9 F8 D9 E1 D8 FE D9 7E E3 83 E3 45 E3 9C E3
DEFW LE3A5, LE3AB, LE3C8, LE3D7, LE3E0, LE3E6, LE3EC, LE3F5 ; D767 D767 A5 E3 AB E3 C8 E3 D7 E3 E0 E3 E6 E3 EC E3 F5 E3
DEFW LE3FE, LE404, LE40A, LE411, LDC2C, LE417, LE41D, LE426 ; D777 D777 FE E3 04 E4 0A E4 11 E4 2C DC 17 E4 1D E4 26 E4
DEFW LE42D, LE441, LE447, LE44D, LE30E, LE453, LDA04, LDA04 ; D787 D787 2D E4 41 E4 47 E4 4D E4 0E E3 53 E4 04 DA 04 DA
DEFW LE49B ; D797 D797 9B E4
LD HL,XD7CA ; D799 D799 21 CA D7
CALL LD7E5 ; D79C D79C CD E5 D7
CP #03 ; D79F D79F FE 03
JP Z,LEC46 ; D7A1 D7A1 CA 46 EC Select ROM 0 and return to BASIC
RET ; D7A4 D7A4 C9
LD7A5: LD HL,XD7D5 ; D7A5 D7A5 21 D5 D7
JP LD7B4 ; D7A8 D7A8 C3 B4 D7
LD7AB: LD HL,XD7E1 ; D7AB D7AB 21 E1 D7
JP LD7B4 ; D7AE D7AE C3 B4 D7
LD7B1: LD HL,XD7DC ; D7B1 D7B1 21 DC D7
LD7B4: CALL LD7E5 ; D7B4 D7B4 CD E5 D7
JP LEC46 ; D7B7 D7B7 C3 46 EC Select ROM 0 and return to BASIC
DSKMSG: DEFS "Bdos Err On " ; D7BA D7BA 42 64 6F 73 20 45 72 72 20 4F 6E 20 Disk error message
DSKERR: DEFS " : $" ; D7C6 D7C6 20 3A 20 24
XD7CA: DEFS "Bad Sector$" ; D7CA D7CA 42 61 64 20 53 65 63 74 6F 72 24
XD7D5: DEFS "Select$" ; D7D5 D7D5 53 65 6C 65 63 74 24
XD7DC: DEFS "File " ; D7DC D7DC 46 69 6C 65 20
XD7E1: DEFS "R/O$" ; D7E1 D7E1 52 2F 4F 24
LD7E5: PUSH HL ; D7E5 D7E5 E5
LD BC,#D80E ; D7E6 D7E6 01 0E D8
CALL LD804 ; D7E9 D7E9 CD 04 D8
LD A,(XDA42) ; D7EC D7EC 3A 42 DA
ADD A,#41 ; D7EF D7EF C6 41
LD (DSKERR),A ; D7F1 D7F1 32 C6 D7
LD BC,DSKMSG ; D7F4 D7F4 01 BA D7
CALL LD804 ; D7F7 D7F7 CD 04 D8
POP BC ; D7FA D7FA C1
CALL LD804 ; D7FB D7FB CD 04 D8
CALL L0079 ; D7FE D7FE CD 79 00
JR Z,LD7FE ; D801 D801 28 FB
RET ; D803 D803 C9
LD804: LD A,(BC) ; D804 D804 0A
CP #24 ; D805 D805 FE 24
RET Z ; D807 D807 C8
CALL L0CAB ; D808 D808 CD AB 0C
INC BC ; D80B D80B 03
JR LD804 ; D80C D80C 18 F6
CRLF0: DEFS "\0D\0A$\00" ; D80E D80E 0D 0A 24 00
POP AF ; D812 D812 F1
RET ; D813 D813 C9
LD814: CP #0D ; D814 D814 FE 0D
RET Z ; D816 D816 C8
CP #0A ; D817 D817 FE 0A
RET Z ; D819 D819 C8
CP #09 ; D81A D81A FE 09
RET Z ; D81C D81C C8
CP #08 ; D81D D81D FE 08
RET Z ; D81F D81F C8
CP #20 ; D820 D820 FE 20
RET ; D822 D822 C9
LD823: LD A,(XDA0E) ; D823 D823 3A 0E DA
OR A ; D826 D826 B7
JP NZ,LD845 ; D827 D827 C2 45 D8
CALL LE506 ; D82A D82A CD 06 E5
AND #01 ; D82D D82D E6 01
RET Z ; D82F D82F C8
CALL LE509 ; D830 D830 CD 09 E5
CP #13 ; D833 D833 FE 13
JP NZ,LD842 ; D835 D835 C2 42 D8
CALL LE509 ; D838 D838 CD 09 E5
CP #03 ; D83B D83B FE 03
JP Z,L0000 ; D83D D83D CA 00 00
CHNL1: DEFB #00, #00 ; D840 D840 00 00 File control block
LD842: DEFB #00, #00, #00 ; D842 D842 00 00 00
LD845: DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D845 D845 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D84D D84D 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D855 D855 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D85D D85D 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00 ; D865 D865 00 00 00 00 00
CHNL2: DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D86A D86A 00 00 00 00 00 00 00 00 File control block
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D872 D872 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D87A D87A 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D882 D882 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00 ; D88A D88A 00 00 00 00 00 00
LD890: DEFB #00, #00, #00, #00 ; D890 D890 00 00 00 00
CHNL3: DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D894 D894 00 00 00 00 00 00 00 00 File control block
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D89C D89C 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8A4 D8A4 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8AC D8AC 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8B4 D8B4 00 00 00 00 00 00 00 00
DEFB #00, #00 ; D8BC D8BC 00 00
CHNL4: DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8BE D8BE 00 00 00 00 00 00 00 00 File control block
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8C6 D8C6 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8CE D8CE 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8D6 D8D6 00 00 00 00 00 00 00 00
DEFB #00, #00, #00 ; D8DE D8DE 00 00 00
LD8E1: DEFB #00, #00, #00, #00, #00, #00, #00 ; D8E1 D8E1 00 00 00 00 00 00 00
CHNL5: DEFB #01, #3F, #3F, #3F, #3F, #3F, #3F, #3F ; D8E8 D8E8 01 3F 3F 3F 3F 3F 3F 3F File control block
DEFB #3F, #3F, #3F, #3F ; D8F0 D8F0 3F 3F 3F 3F
LINPUT: DEFB #00, #00 ; D8F4 D8F4 00 00
IXTEMP: DEFB #00, #00 ; D8F6 D8F6 00 00
STORE: DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D8F8 D8F8 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D900 D900 00 00 00 00 00 00 00 00
DEFB #00, #00, #00, #00, #00, #00, #00, #00 ; D908 D908 00 00 00 00 00 00 00 00
DEFB #00, #00 ; D910 D910 00 00