...
The Mpm assembler generates the relocation table, when parsing the source code for compilation of the binary executable code. The relocator routine that is used by Mpm is made available here for convenience:
| Code Block | ||||
|---|---|---|---|---|
| ||||
MODULE relocator XDEF relocator, SIZEOF_relocator ; ************************************************************************* ; ; This routine will relocate a machine code program. ; A relocation table is placed immediatly after the relocation routine. ; ; The program that is to be relocated and executed are placed immediately ; after the relocation table. ; ; The relocation routine expects IY to point at the start address, entry ; of the relocater. The relocater then calculates the correct origin for ; the relocated program. The BBC BASIC on the Z88 also setups the IY register ; during a BBC BASIC CALL command to execute local machine code. ; ; After completed relocation, the entry of the relocater is patched to ; JP which jumps directly to the executing program, if the relocated ; code is called again. ; ; The following registers are affected by the initial relocation process: ; AFBCDEHL/IX../........ same ; ......../..IY/afbcdehl different ; ; As indicated above the outside world cannot use the alternate registers ; as parameter interface to the relocated program - they are smashed by the ; relocater. ; .relocator ex af,af' ex af,af' ; preserve AF ; preserve AF exx ; preserve BC, DE, HL push iy pop pushhl iy ld bc, #end_relocator-relocator popadd hl,bc ; ldabsolute address of bc, #end_relocator-relocatorrelocation table ld e,(hl) addinc hl,bc ld d,(hl) ;push absolutede address of relocation table ; DE = ldtotal of relocation e,(hl)offset elements inc hl ld inc c,(hl) inc hl ld db,(hl) ; total pushsize deof relocation offset elements inc hl ;push DEhl = total of relocation offset elements ; preserve pointer to incfirst relocation hloffset element add hl,bc ld cb,(hl)h inc ; hlHL = pointer to current relocation address ld c,l ld b,(hl) ; BC = ;program total sizeORG, first byte of relocationprogram offset elements.relocate_loop ex (sp),hl inc hl ; HL = pointer to relocation offset element push hl ld a,(hl) inc hl ; preserve pointer to first relocation offset element ; ready for next relocation offset pointer add hl,bcor a jr ld b,h ; HL = pointer to current relocation address ld c,l ; BC = program ORG, first byte of program .relocate_loop ex (sp),hl ; HL = pointer to relocation offset element ld a,(hl) inc hl ; ready for next relocation offset pointer or a jr nz, byte_offset .extended_offset ld e,(hl) inc hl ld d,(hl) ; DE = extended offset pointer to next relocation address inc hl ; ready for next relocation offset pointer jr relocate_address .byte_offset ld d,0 ld e,a ; offset pointer to next relocation address .relocate_address ex (sp),hl ; HL = pointer to current relocation address add hl,de ; new pointer at memory that contains relocation address nz, byte_offset .extended_offset ld e,(hl) inc hl ld d,(hl) ; DE = extended offset pointer ex de,hl to next relocation address inc hl add hl,bc ; ready for next relocation offset ;pointer HL = address relocated to program ORG in BCjr relocate_address .byte_offset ld d,0 exld dee,hla ; offset pointer to ldnext relocation (hl),d address .relocate_address ex (sp),hl dec hl ; HL = pointer to current relocation address add hl,de ld (hl),e ; new pointer at memory that ;contains update relocatedrelocation address back to memory ld e,(hl) inc hl pop deld d,(hl) ex de,hl ; DEadd = pointerhl,bc to relocation offset ; HL = address relocated to program ORG in exBC (sp),hl ex de,hl ;ld HL = index(hl),d counter dec hl ld (hl),e dec hl ; update relocated address back to memory ; update index counter pop de ld a,h; DE = pointer to relocation offset ex (sp),hl or l ; HL = index counter dec hl ; all addresses relocated? ; update index counter ex (sp),hl ld a,h or l ; index counter back on stack ; all addresses relocated? push de ex (sp),hl ; pointerindex tocounter relocation offset back on stack push de jr nz, relocate_loop ; pointer to relocation offset back on stack pop af jr nz, relocate_loop pop af pop af pop af ; remove redundant variables ; relocation of program completed. Patch the entry of the relocater to JP that ; jumps directly to the executing program, if the loaded program is executed again. ; Finish with restoring main registers and then execute program. ; .relocation_finished ld (iy+0),$C3 ld (iy+1),c ld (iy+2),b ; patch entry to JP , the relocated program exx ; swap back to main BC, DE, HL ex af,af' ; and main AF jp (iy) ; execute relocated program... .end_relocator DEFC SIZEOF_relocator = end_relocator - relocator ; ****************************************************************************** ; ; The relocation table is placed here by the Mpm assembler. ; The format of the generated table is: ; ; total_elements ds.w 1 ; sizeof_table ds.w 1 ; patchpointer_0 ds.b 1 --+ ; patchpointer_1 ds.b 1 | ; .... | sizeof_table ; .... | ; patchpointer_n ds.b 1 --+ ; ; The first patch pointer is an offset from the start of the program (.routine) ; to the first position of a location that contains a relocatable address. ; The following patchpointers are relative offsets from the current relocated ; address to the next. ; If the offset distance is larger than 255 bytes between two relocatable ; addresses, the following patchpointer is used: ; ; 0,<16bit patchpointer> ; ; which denotes the an offset from the current relocated address to the next. ; The 16 bit patch pointer is stored in the low byte, high byte order. ; .routine ; the machine code to be relocated is placed immediately after the relocation table |