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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:
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MODULE relocatorXDEFrelocator 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) ; absolutepush addressde of relocation table ; DE ld= total of relocation offset e,(hl)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 c,(hl)b,h inc; HL hl= pointer to current relocation address ld c,l ld b,(hl) ; BC = program ;ORG, totalfirst sizebyte 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 nz, byte_offset .extended_offset ld b,h ld e,(hl) ; HLinc = pointerhl to current relocation address ld d,(hl) ld c,l ; DE = extended offset pointer to next relocation address ;inc BC =hl program ORG, first byte of program .relocate_loop ex (sp),hl ; ready for next relocation offset pointer jr ; HLrelocate_address = pointer to relocation .byte_offset element ld d,0 ld e,a ld a,(hl) ; offset pointer to next relocation address .relocate_address inc hl ex (sp),hl ; HL = readypointer forto nextcurrent relocation offsetaddress pointer add hl,de or a ; new pointer at memory that contains relocation address ld e,(hl) jr nz, byte_offset .extended_offset inc hl ld ed,(hl) ex de,hl add hl,bc inc hl ; HL = address relocated to program ORG in BC ld d,(hl) ex de,hl ld ;(hl),d DE = extended offset pointer to next relocation addressdec hl ld (hl),e inc hl ; update relocated address back to memory pop ;de ready for next relocation offset pointer ; DE = pointer to relocation offset jr relocate_address .byte_offset ex (sp),hl ld d,0 ; HL = index counter dec hl ld e,a ; update index counter ; offset pointer to next relocationld address .relocate_address a,h ex or (sp),hl l ; HL = pointer to current relocation address ; all addresses relocated? ex (sp),hl add hl,de ; index counter back on stack ;push newde pointer at memory that contains relocation address ; pointer to relocation offset back on stack ld e,(hl) jr nz, relocate_loop pop af inc pop hl af ; remove ldredundant variables d,(hl); relocation of program completed. Patch the entry of the relocater to JP that ; jumps directly to the executing program, if the loaded program exis executed again. de,hl; Finish with restoring main registers and then execute program. ; .relocation_finished ld add(iy+0),$C3 hl,bc ld (iy+1),c ld ; HL = address relocated to program ORG in BC (iy+2),b ; patch entry to JP , the relocated program exexx de,hl ; swap ldback to main (hl)BC,d DE, HL ex af,af' dec hl ; and main AF jp (iy) ld (hl),e ; updateexecute relocated address back to memory pop de ; DE = pointer to relocation offset ex (sp),hl ; HL = index counter dec hl ; update index counter ld a,h or l ; all addresses relocated? ex (sp),hl ; index counter back on stack push de ; pointer to relocation offset back on stack jr nz, relocate_loop pop af pop af ; remove redundant variables ; relocation of program completed. Patch the entry of the relocater to JP that ; jumps directly to the executing program, if the loaded program is executed again. ; Finish with restoring main registers and then execute program. ; .relocation_finished ld (iy+0),$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 |
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program... .end_relocator DEFC SIZEOF_relocator = end_relocator - relocator ; ****************************************************************************** |
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;
; 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 |