This is a very barebones CP/M port for the Agon Light, using the BBC BASIC assembler to do all the work.

cpm.bas

   10 bbase=&FD00
   20 fbase=bbase-3584
   30 cbase=fbase-2048
   40 iobyte=&0003
   50 cdisk=&0004
   60 bdoscall=&0005
   70 bioscall=&0000
   80 DIM code 4096
   90 FOR pass=4 TO 7 STEP 3
  100   P%=0
  110   O%=code
  120   [OPT pass
  130   :
  140   ; mos binary header and startup code
  150   :
  160   jp main
  170   .loop
  180   DEFM "12345"
  190   :
  200   ]
  210   FOR r=&08 TO &30 STEP 8
  220     [OPT pass
  230     DEFB &49
  240     rst r
  250     ret
  260     DEFM "12345"
  270     ]
  280   NEXT
  290   [OPT pass
  300   :
  310   ei
  320   reti
  330   DEFM "12345"
  340   :
  350   DEFM "MOS"
  360   DEFB 0
  370   DEFB 0
  380   :
  390   .startup_banner
  400   DEFB 12
  410   DEFM "Hacked up CP/M for the Agon Light"
  420   DEFB 13
  430   DEFB 10
  440   .startup_banner_end
  450   .main
  460   ld sp, &8000
  470   ld hl, startup_banner
  480   ld bc, startup_banner_end - startup_banner
  490   DEFB &49
  500   rst &18 ; print string
  510   :
  520   ld hl, bios_load_address
  530   ld de, bbase
  540   ld bc, &10000-bbase
  550   ldir
  560   :
  570   ld hl, diskimage_filename
  580   ld c, 3
  590   ld a, &A ; mos_fopen
  600   DEFB &49
  610   rst 8
  620   ld (filehandle), a
  630   :
  640   jp bbase
  650   :
  660   .diskimage_filename
  670   DEFM "cpmdisk.img"
  680   DEFB 0
  690   ]
  700   :
  710   bios_load_address=P%
  720   P%=bbase
  730   [opt pass
  740   jp boote
  750   jp wboote
  760   jp conste
  770   jp conine
  780   jp conoute
  790   jp liste
  800   jp punche
  810   jp readere
  820   jp homee
  830   jp seldske
  840   jp settrke
  850   jp setsece
  860   jp setdmae
  870   jp reade
  880   jp writee
  890   jp listste
  900   jp sectrane
  910   :
  920   .boote
  930   xor a
  940   ld (iobyte), a
  950   ld (cdisk), a
  960   ; fall through
  970   :
  980   .wboote
  990   ld sp, &0100
 1000   ld a, &C3
 1010   ld (bdoscall), a
 1020   ld hl, fbase+6
 1030   ld (bdoscall+1), hl
 1040   ld (bioscall), a
 1050   ld hl, bbase+3
 1060   ld (bioscall+1), hl
 1070   ld hl, bdosccp_filename
 1080   ld de, cbase
 1090   ld bc, bbase-cbase
 1100   ld a, 1 ; mos_load
 1110   DEFB &49
 1120   rst 8
 1130   ld a, (cdisk)
 1140   ld c, a
 1150   jp cbase
 1160   .bdosccp_filename
 1170   DEFM "bdosccp.img"
 1180   DEFB 0
 1190   :
 1200   .conste
 1210   xor a
 1220   ret
 1230   :
 1240   .conine
 1250   ld a, 0 ; mos_getkey
 1260   DEFB &49
 1270   rst 8
 1280   or a
 1290   jr z, conine
 1300   ret
 1310   :
 1320   .conoute
 1330   ld a, c
 1340   DEFB &49
 1350   rst &10
 1360   ret
 1370   :
 1380   .liste
 1390   .punche
 1400   .readere
 1410   ret
 1420   :
 1430   .homee
 1440   ld hl, 0
 1450   ld (current_track), hl
 1460   ld (current_sector), hl
 1470   ret
 1480   :
 1490   .seldske
 1500   ld a, c
 1510   ld (current_drive), a
 1520   ld hl, dph
 1530   or a
 1540   ret z
 1550   ld hl, 0
 1560   ret
 1570   .current_drive
 1580   DEFB 0
 1590   :
 1600   .settrke
 1610   ld (current_track), bc
 1620   ret
 1630   .current_track
 1640   DEFW 0
 1650   :
 1660   .setsece
 1670   ld (current_sector), bc
 1680   ret
 1690   .current_sector
 1700   DEFW 0
 1710   :
 1720   .setdmae
 1730   ld (current_dma), bc
 1740   ret
 1750   .current_dma
 1760   DEFW 0
 1770   :
 1780   .prepare_read_write
 1790   call seek_to_sector
 1800   ld hl, (current_dma)
 1810   ld a, &04
 1820   call set_hlu
 1830   DEFB &5B: ld de, 128: DEFB 0
 1840   ret
 1850   :
 1860   .reade
 1870   call prepare_read_write
 1880   ld a, &1A ; mos_fread
 1890   DEFB &49
 1900   rst 8
 1910   .check_read_write_result
 1920   ld hl, -128
 1930   add hl, de
 1940   ld a, h
 1950   or l
 1960   ret z
 1970   ld a, 1
 1980   ret
 1990   :
 2000   .writee
 2010   call prepare_read_write
 2020   ld a, &1B ; mos_fwrite
 2030   DEFB &49
 2040   rst 8
 2050   jr check_read_write_result
 2060   :
 2070   .listste
 2080   xor a
 2090   ret
 2100   :
 2110   .sectrane
 2120   ld h, b
 2130   ld l, c
 2140   ret
 2150   :
 2160   .seek_to_sector
 2170   ld a, (current_track)
 2180   ld h, a
 2190   ld a, (dpb+0)
 2200   ld l, a
 2210   mlt hl
 2220   ld de, (current_sector)
 2230   add hl, de
 2240   xor a
 2250   srl h
 2260   rr l
 2270   rra
 2280   ld c, a
 2290   ld a, h
 2300   ld h, l
 2310   ld l, c
 2320   call set_hlu
 2330   ld e, 0
 2340   ld a, (filehandle)
 2350   ld c, a
 2360   ld a, &1C ; mos_flseek
 2370   DEFB &49
 2380   rst 8
 2390   ret
 2400   :
 2410   .set_hlu
 2420   ld (hlu_insn+2), hl
 2430   ld (hlu_insn+4), a
 2440   .hlu_insn
 2450   DEFB &5B: ld hl, 0: DEFB 0
 2460   ret
 2470   :
 2480   .phex8
 2490   ld a, c
 2500   rra
 2510   rra
 2520   rra
 2530   rra
 2540   call phexconv
 2550   ld a, c
 2560   .phexconv
 2570   and &0F
 2580   add a, &90
 2590   daa
 2600   adc a, &40
 2610   daa
 2620   DEFB &49
 2630   rst &10
 2640   ret
 2650   .dph
 2660   DEFW 0 ; sector translation vector
 2670   DEFW 0 ; BIOS scratchpad 1
 2680   DEFW 0 ; BIOS scratchpad 2
 2690   DEFW 0 ; BIOS scratchpad 3
 2700   DEFW dirbuf ; directory buffer
 2710   DEFW dpb ; DPB for this drive
 2720   DEFW drive_a_check_vector
 2730   DEFW drive_a_bitmap
 2740   .dpb
 2750   DEFW 18*4 ; CP/M sectors per track
 2760   DEFB 4 ; BSH
 2770   DEFB 15 ; BLM
 2780   DEFB 0 ; EXM
 2790   DEFW 350 ; DSM, number of blocks -1
 2800   DEFW 127 ; DRM, number of dirents -1
 2810   DEFW &00C0 ; initial allocation vector
 2820   DEFW 32 ; size of disk check vector
 2830   DEFW 2 ; number of reserved tracks
 2840   .drive_a_bitmap
 2850   OPT FNadvance(44)
 2860   .drive_a_check_vector
 2870   OPT FNadvance(32)
 2880   .dirbuf
 2890   OPT FNadvance(128)
 2900   .filehandle
 2910   DEFB 0
 2920   ]
 2930   :
 2940 NEXT
 2950 len=O%-code
 2960 PRINT "Code length: ";len
 2970 DIM savecode 40
 2980 P%=savecode
 2990 [
 3000 .filename
 3010 DEFM "CPM.BIN"
 3020 DEFB 0
 3030 .saver
 3040 :  ld hl, filename
 3050 :  ld a, 5 ; mos_del
 3060 DEFB &49
 3070 :  rst 8
 3080 :  ld hl, filename
 3090 :  ld de, code
 3100 :  ld bc, len
 3110 :  ld a, 2 ; mos_save
 3120 DEFB &49
 3130 :  rst 8
 3140 :  ld l, a
 3150 :  ld h, 0
 3160 :  ret
 3170 ]
 3180 L% = (USR saver) / &10000
 3190 IF L% THEN PRINT "Save failed! ";~L%:STOP
 3200 PRINT "cbase=";~cbase
 3210 PRINT "fbase=";~fbase
 3220 PRINT "bbase=";~bbase
 3230 SAVE "cpm.bas"
 3240 END
 3250 :
 3260 DEF FNadvance(amount)
 3270 P%=P%+amount
 3280 O%=O%+amount
 3290 =pass