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NES memory map amend (16k prg, 8k chr default) 

The configuration file and runtime (crt0.s) provided for the default NES
ROM layout (2x16k PRG, 8k CHR) incorrectly added interrupts (IRQ1, IRQ2,
TIMERIRQ) which are not supported by the NES hardware.  For example, see
the NESdev wiki, which makes no reference to these interrupts.

    https://wiki.nesdev.com/w/index.php/CPU_memory_map

The VECTORS region was also incorrectly set to 0xFFF6, which would have
left the 0xFFF4 normally unspecified.  This did not result in any error,
however, since cc65 simply placed ROMV directly after ROM0 regardless of
start address.

(This layout may be due to a copy-and-paste from the PC-Engine
configuration, whose interrupt registers start at 0xFFF6, begins with
the three interrupts listed above, followed by NMI and START, and does
not end with a final IRQ interrupt.)

Despite the absence of any actual error, since START is still placed at
0xFFFC, this patch removes the nonexistent interrupts and also correctly
aligns the ROM0 and ROMV regions.  It also has the (admittedly very
minor) benefit of freeing up 6 additional bytes for ROM0.
This commit is contained in:
Marshall Ward 2016-12-20 22:12:08 +11:00
parent 4b78364b8b
commit 09495519c0
2 changed files with 2 additions and 8 deletions
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4
cfg/nes.cfg
View File

@ -12,10 +12,10 @@ MEMORY {
# - code
# - rodata
# - data (load)
ROM0: file = %O, start = $8000, size = $7FF4, fill = yes, define = yes;
ROM0: file = %O, start = $8000, size = $7FFA, fill = yes, define = yes;
# Hardware Vectors at End of 2nd 8K ROM
ROMV: file = %O, start = $FFF6, size = $000C, fill = yes;
ROMV: file = %O, start = $FFFA, size = $0006, fill = yes;
# 1 8k CHR Bank
ROM2: file = %O, start = $0000, size = $2000, fill = yes;

6
libsrc/nes/crt0.s
View File

@ -159,9 +159,6 @@ nmi: pha
; Interrupt exit
irq2:
irq1:
timerirq:
irq:
rti
@ -171,9 +168,6 @@ irq:
.segment "VECTORS"
.word irq2 ; $fff4 ?
.word irq1 ; $fff6 ?
.word timerirq ; $fff8 ?
.word nmi ; $fffa vblank nmi
.word start ; $fffc reset
.word irq ; $fffe irq / brk