bacldr: Add loader variant that supports relocatable code

Add a version of the loader which supports relocatable code to be
loaded at BOFA.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 65 insertions, 0 deletions

diff --git a/data/bacldr2.asm b/data/bacldr2.asm
new file mode 100644
index 0000000..ae9c381
--- /dev/null
+++ b/data/bacldr2.asm
@@ -0,0 +1,65 @@
+	;; Load an assembly program at BOFA and handle relocations
+	;; Relocations are a list of 16-bit addresses at the end of
+	;; the image, processed in reverse order, terminated with
+	;; an FFFFh sentinel.
+
+	org 0			; This code must be position-independent
+
+BOFA:	equ 65052
+offset:	equ 0xeeee		; Length of the BASIC prefix
+ENTRY:	equ 0xbbbb
+
+	ld hl,(BOFA)
+	push hl
+	ld e,l
+	ld d,h
+	ld bc,offset		; Offset < 256 so B = 0
+	add hl,bc
+
+	; Get data block length
+loop:
+	ld a,-8
+	add (hl)
+	jr nc,reloc		; END is a shorter line than any data line
+
+	; Skip length byte + line number (2 bytes) + string opcode (3 bytes)
+	ld c,6
+	add hl,bc
+
+	; Copy remaining data
+	ld c,a
+	ldir
+
+	; Skip return + CR
+	inc hl
+	inc hl
+	jr loop
+
+	;; Process relocations.  At this point DE points beyond
+	;; the end of the contiguous image.
+reloc:
+	pop bc			; BC <- BOFA
+reloc_loop:
+	ex de,hl
+	dec hl
+	ld d,(hl)
+	dec hl
+	ld e,(hl)
+	ld a,e
+	and d
+	inc a			; A = FFh only if BC = FFFFh
+	jr z,reloc_done
+	ex de,hl
+	add hl,bc
+	ld a,(hl)
+	add c
+	ld (hl),a
+	inc hl
+	ld a,(hl)
+	adc a,b
+	ld (hl),a
+	jr reloc_loop
+reloc_done:
+	ld hl,ENTRY
+	add hl,bc
+	jp (hl)