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These are some development notes I've put together that would be of great
aid to those willing to contribute to the PSn00bSDK project. Many of these
came from my own experience dealing with the PS1 at low-level when I ran
into some unexplained quirks while some are from disassembly observations
and clarification of existing documents. More entries will be added when
I run into more previously undocumented quirks in the future.
- Lameguy64
* When calling C functions (ie. BIOS functions) from assembly code you'll
need to allocate N words on the stack first when calling a function that has
N arguments (addiu $sp, -(4*N) where N = number of arguments of the function
being called) even if the arguments are on registers a0 to a3 and the C
functions don't always use the space allotted in stack. When calling a
function with a variable number of arguments (printf) always allocate
16 bytes of stack.
* Hooking a custom handler using BIOS function HookEntryInt (B(19h), known
as SetCustomExitFromException in nocash docs) is only triggered when there's
an IRQ that is not yet acknowledged by previous IRQ handlers built into the
kernel. This is also the best point to acknowledge any IRQs without breaking
compatibility with built-in BIOS IRQ handlers and is what the official SDK
uses to handle IRQs. To make sure this handler is triggered on every interrupt
you must call ChangeClearPad(0) and ChangeClearRCnt(3, 0) (which are functions
(B(5Bh)) and C(0Ah) respectively) otherwise the pad and root counter handlers
in the kernel will acknowledge the interrupt before your handler, preventing
you from handling them yourself.
* It is not advisable to handle interrupts using event handlers like in
PSXSDK as it breaks BIOS features that depend on interrupts. Clearing the
VBlank IRQ in a event handler for example prevents the BIOS controller
functions from working as it depends on the VBlank IRQ to determine when to
query controllers. Acknowledge interrupts using a custom handler set by BIOS
function HookEntryInt (B(19h), known as SetCustomExitFromException in nocash
docs).
* When running in high resolution mode you must additionally wait for bit 31
in GPUSTAT (1F801814h) to alternate on every frame (frame 0: wait until 0,
frame 1: wait until 1, frame 2: wait until 0) before waiting for VSync
otherwise the GPU will only draw the first field if you don't have drawing
to displayed area enabled. Performing this check in a low resolution/non
interlaced mode is harmless.
* There's a hardware bug in the GPU FillVRAM command GP0(02h) where if you
set the height to 512 pixels the primitive is processed with a height of 0
as the hardware does not appear to interpret the last bit of the height
value. This is most apparent when putting a DRAWENV with the height of 512
pixels (for PAL for example) and background clearing is enabled, hence why
DRAWENV.isbg is not effective in the official SDK.
* In the official SDK, DMA IRQs appear to be enabled only when a callback
function is set (ie. DrawSyncCallback() enables IRQ for DMA channel 2). DMA
IRQs are only triggered on transfer completion.
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