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Adapted, to the c64 target, the INIT-segment overlay scheme from the apple2 targets.

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When a program starts running, INIT is moved from one place to another place.  Then, INIT's code is executed; and, the first place is re-used for variables.  After the INIT code has finished, the second place can be re-used by the heap and the C stack.  That means that initiation code and data won't waste any RAM space after they stop being needed.
This commit is contained in:
Greg King
2015-10-05 05:47:43 -04:00
parent 24c2da980b
commit 074e10d288
5 changed files with 200 additions and 104 deletions
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101
libsrc/c64/crt0.s
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@@ -3,13 +3,13 @@
;
.export _exit
.exportzp init_load_, init_run_
.export __STARTUP__ : absolute = 1 ; Mark as startup
.import initlib, donelib
.import zerobss
.import callmain
.import move_init, zerobss, callmain
.import RESTOR, BSOUT, CLRCH
.import __RAM_START__, __RAM_SIZE__ ; Linker generated
.import __STACKSIZE__ ; Linker generated
.import __HIMEM__ ; from configure file
.importzp ST
.include "zeropage.inc"
@@ -19,18 +19,17 @@
; ------------------------------------------------------------------------
; Startup code
; Two zero-page pointers are needed before any zero-page stuff is saved.
; Choose locations that are not used by anything.
init_load_ := FREKZP
init_run_ := FREKZP+2
.segment "STARTUP"
Start:
; Save the zero-page locations that we need.
ldx #zpspace-1
L1: lda sp,x
sta zpsave,x
dex
bpl L1
; Switch to the second charset.
lda #14
@@ -39,35 +38,30 @@ L1: lda sp,x
; Switch off the BASIC ROM.
lda $01
pha ; Remember the value
sta mmusave ; Save the memory configuration
and #$F8
ora #$06 ; Enable Kernal+I/O, disable BASIC
sta $01
; Clear the BSS data.
jsr zerobss
; Save some system settings; and, set up the stack.
pla
sta mmusave ; Save the memory configuration
tsx
stx spsave ; Save the system stack ptr
lda #<(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__)
sta sp
lda #>(__RAM_START__ + __RAM_SIZE__ + __STACKSIZE__)
sta sp+1 ; Set argument stack ptr
; Allow some re-entrancy by skipping the next task if it already was done.
; This often can let us rerun the program without reloading it.
; Call the module constructors.
ldx moveinit
beq L0
jsr initlib
; Move the INIT segment from where it was loaded (over ZPSAVE and BSS)
; into where it must be run (in the heap).
; Push the command-line arguments; and, call main().
jsr move_init
dec moveinit ; set to false
jsr callmain
; Save space by putting the rest of the start-up code in the INIT segment,
; which can be re-used by the heap.
L0: jsr initstart
; Back from main() [this is also the exit() entry]. Run the module destructors.
@@ -98,14 +92,51 @@ L2: lda zpsave,x
rts
; ------------------------------------------------------------------------
.segment "INIT"
initstart:
; Save the zero-page locations that we need.
ldx #zpspace-1
L1: lda sp,x
sta zpsave,x
dex
bpl L1
; Clear the BSS data.
jsr zerobss
; Set up the stack.
lda #<__HIMEM__
ldx #>__HIMEM__
sta sp
stx sp+1 ; Set argument stack ptr
; Call the module constructors.
jsr initlib
; Push the command-line arguments; and, call main().
jmp callmain
; ------------------------------------------------------------------------
; Data
.data
mmusave:.res 1
spsave: .res 1
moveinit:
.byte 1
.segment "ZPSAVE"
zpsave: .res zpspace
.bss
spsave: .res 1
mmusave:.res 1