This is a document intended to describe various aspects of SNES timing. It will probably not be useful unless you already know a good bit about the SNES.

BTW, special credit to byuu for the critical observation that the SNES returns to a known timing position on reset. Thus, a deterministic ROM (i.e. it doesn't depend on user input or any other randomness) will always give the same results on reset. And a series of ROMs which vary only in the master cycle count before testing some event (like the value of $4212) can tell us just when such events occur.

A note on timings: stating that a bit set is at H=X means that a read that would latch X-.5 were it reading $2137 will see the bit not set, while a read that would latch X were it reading $2137 will see the bit set. The counter may also be latched by writing 0 to $4201 bit 7: this will latch 1 dot later than if the same memory access cycle were reading $2137.

  • S-CPU (5A22)
    • Clocks & Refresh
    • Instructions
    • Interrupts
    • DMA
    • HDMA
    • Auto Joypad Read
    • Registers
  • S-PPU
    • Rendering
    • Blanking periods
    • Registers
    • OAM Reset
  • S-APU
    • SPC700
    • DSP

S-CPU (5A22)

Clocks & Refresh

The SNES master clock runs at about 21.477MHz NTSC (theoretically 1.89e9/88 Hz). The best number we have for PAL is 21.28137MHz.

A CPU internal operation (an IO cycle) takes 6 master cycles. A memory access cycle takes 6, 8, or 12 master cycles, depending on the memory region accessed and bit 0 of CPU register $420d.

The SNES runs 1 scanline every 1364 master cycles, except in non-interlace mode scanline $f0 of every other frame (those with $213f.7=1) is only 1360 cycles. Frames are 262 scanlines in non-interlace mode, while in interlace mode frames with $213f.7=0 are 263 scanlines. "V-Blank" runs from either scanline $e1 or $f0 until the end of the frame.

The CPU is paused for 40 cycles beginning about 536 cycles after the start of each scanline. Current theory is that this is used for WRAM Refresh. The exact timing is that the refresh pause begins at 538 cycles into the first scanline of the first frame, and thereafter some multiple of 8 cycles after the previous pause that comes closest to 536.

Instructions

For specifics on particular instructions, see any generic 65816 doc. The GTE datasheet is particularly nice, as it identifies the CPU activity for each cycle of the instruction.

To determine the exact length of any CPU instruction, you must examine its behavior for each cycle, and count 6, 8, or 12 master cycles as appropriate.

The WAI instruction stops the processor. The processor restarts when either the /NMI or /IRQ line is low (or /RESET, but we don't care about that too much). It takes 12 master cycles (2 IO cycles) to end the WAI instruction, at which point the NMI or IRQ handler may actually be executed.

Interrupts

The internal timer will set its NMI output low at H=0.5 at the beginning of V-Blank. The CPU's /NMI input is forced high by clearing bit 7 of register $4200, so the CPU may not actually see the NMI transition. The CPU will jump to the NMI routine at the end of the instruction during which /NMI transitions.

The internal timer sets its NMI output high at H=0 V=0, or when register $4210 is read. Possibly also when $4200 is written?

If the CPU is halted (i.e. for DMA) while /NMI goes low, the NMI will trigger after the DMA completes (even if /NMI goes high again before the DMA completes). In this case, there is a 24-30 cycle delay between the end of DMA and the NMI handler, time enough for an instruction or two.

The internal timer will set its IRQ output low under the following conditions ('x' and 'y' are bits 4 and 5 of $4200, HTIME is registers $4207-8, and VTIME is $4209-a):

yx    trigger point
00 => Never
01 => H-IRQ:  every scanline, H=HTIME+~3.5
10 => V-IRQ:  V=VTIME, H=~2.5
11 => HV-IRQ: V=VTIME, H=HTIME+~3.5

The actual formula for the trigger point is as follows. V-IRQ is just like HV-IRQ with H=0. If H=0, $4211 bit 7 gets set 1374 master cycles after dot 0.0 of the previous scanline. Otherwise, it gets set 14+H*4 master cycles after dot 0.0 of the current scanline. Note that the 'dot offset' will change due to the two long dots per scanline. Also, no IRQ will trigger for dot 153 on the short scanline in non-interlace mode, and no IRQ will trigger for dot 153 on the last scanline of any frame.

The internal timer will set its IRQ output high when $4211 is read, or when IRQs are disabled by a write to $4200. Note that the expansion port and the cart connector both have access to the /IRQ line, and may be able to trigger IRQs on their own. When enabling IRQs, the IRQ output will go low even if the enable write occurs at the exact cycle when the IRQ is scheduled to trigger For example, if HV-IRQ is set for (0,1) and the last cycle of the STA $4200 is at (0,0)+1372 master cycles, the IRQ line will still go low.

The CPU will jump to the NMI or IRQ handler at the end of the instruction when /NMI transitions or when /IRQ is low and the I flag is clear. The actual check occurs just before the final CPU cycle of the instruction, which means that the jump will begin at the earliest 6 to 12 master cycles after /NMI or /IRQ. Also note that PLP, CLI, SEI, SEP #$04, and REP #$04 update the flags during their final CPU cycle, so the IRQ check will use the old value of I rather than the new one set by the current instruction. RTI, BRK, and COP on the other hand do not have this issue.

So for the following code:

>     ; set up IRQ
>     SEI
>     WAI
>     STZ $00
>     LDA #$01
>     CLI
>     LDA #$42
>     STP
> 
> IRQHandler:
>     STA $00
>     RTI

Memory location $00 will end up set to 0x42, not 0 or 1, because the I flag isn't clear before the final cycle of CLI. And for the following code:

>     ; set up IRQ
>     SEI
>     WAI
>     CLI
>     SEI

The IRQ will actually trigger following the SEI instruction, not before it (but the flags pushed during the IRQ handler will have the I flag set). OTOH, the following code will not allow an IRQ to trigger at all if the RTI sets the I flag:

>     ; set up IRQ
>     SEI
>     WAI
>     CLI
>     RTI

And the following code (with RTI clearing I):

>     ; set up IRQ
>     SEI
>     WAI
>     STZ $00
>     LDA #$01
>     RTI
>
>     ; -> RTI returns here
>     LDA #$42
>     STP
> 
> IRQHandler:
>     STA $00
>     RTI

Will result in memory location $00 being set to 1, not 0x42.

If /NMI and /IRQ are both pending, NMI takes precedence.

And the datasheet is inaccurate regarding that first cycle of the IRQ/NMI pseudo-opcode. It's an opcode fetch cycle from PB:PC (typically 6 or 8 master cycles), not an IO cycle (always 6 master cycles) as the datasheet claims.

DMA

DMA is activated by writing a 1 to any bit of CPU register $420b. The CPU is halted during the DMA transfer.

DMA takes 8 master cycles per byte transferred, regardless of the memory regions accessed. It is unknown what happens if you attempt DMA to or from $4000-$41ff, since that memory region requires 12 master cycles for access. There is also 8 master cycles overhead per channel.

The exact DMA timing works as follows: After $420b is written, the CPU gets one more CPU cycle before the pause (on a standard "STA $420b", this would be the opcode fetch for the next instruction). The speed of the next CPU cycle to execute after the DMA determines the CPU Clock speed for the DMA transfer.

Now, after the pause, wait 2-8 master cycles to reach a whole multiple of 8 master cycles since reset. The perform the DMA: 8 master cycles overhead and 8 master cycles per byte per channel, and an extra 8 master cycles overhead for the whole thing. Then wait 2-8 master cycles to reach a whole number of CPU Clock cycles since the pause, and only then resume S-CPU execution.

The exact timing of the read within the DMA period is not known. Best guess at this point is that 2-4 of the "whole DMA overhead" is before the transfer and the rest after.

An example: "STA $420b : NOP", one channel active for a 3 byte transfer. The pause occurs after the NOP opcode is fetched, so the CPU Clock Speed is 6 master cycles due to the following IO cycle in the NOP. After the pause, say we need 2 master cycles to reach an even multiple of 8 master cycles since reset [total=2]. Then wait 8 for DMA init [total=10], 8 for channel init [total=18], and 8*3 for the actual transfer [total=42]. To reach a whole number of CPU Clock cycles since the pause, we must wait 6 master cycles (remember, 0 is not an option) [total=48].

Same thing, but begin the pause 2 master cycles earlier. Thus, we must wait 4 master cycles to get to a multiple of 8 since reset [total=4], then our 40 for the DMA transfer [total=44]. To get to a whole CPU Clock, 4 master cycles are needed [total=48].

Same thing, but begin the pause 2 master cycles earlier. Thus, we must wait 6 master cycles to get to a multiple of 8 since reset [total=6], then our 40 for the DMA transfer [total=46]. To get to a whole CPU Clock, only 2 master cycles are needed [total=48].

One last time, begin the pause 2 master cycles earlier. Thus, we must wait 8 master cycles to get to a multiple of 8 since reset [total=8], then our 40 for the DMA transfer [total=48]. To get to a whole CPU Clock sinze pause, 6 master cycles are again needed [total=54]. So this transfer took an extra 6 master cycles...

HDMA

HDMA is enabled by writing a 1 to any bit of CPU register $420c. Much like DMA, the CPU is halted during HDMA operations. HDMA takes priority over DMA.

For all active channels, the HDMA registers are initialized at about V=0 H=6. The overhead is ~18 master cycles, plus 8 master cycles for each channel set for direct HDMA and 24 master cycles for each channel set for indirect HDMA. Presumably, the exact timing for the HDMA pause is the same as that for DMA.

HDMA channels may be deactivated mid-frame if $00 is read into $43xA from the HDMA table (or possibly if $00 is written to $43xA manually). They may also be activated or deactivated by writing $420c. All HDMA channels are deactivated at the start of V-Blank.

The actual HDMA transfer begins at dot 278 of the scanline (or just after, the current CPU cycle is completed before pausing), for every visible scanline (0-224 or 0-239, depending on $2133 bit 3). For each scanline during which HDMA is active (i.e. at least one channel has not yet terminated for the frame), there are ~18 master cycles overhead. Each active channel incurs another 8 master cycles overhead for every scanline, whether or not a transfer actually occurs. If a new indirect address is required, 16 master cycles are taken to load it. Then 8 cycles per byte transferred are used.

Auto Joypad Read

When enabled, the SNES will read 16 bits from each of the 4 controller port data lines into registers $4218-f. This begins between dots 32.5 and 95.5 of the first V-Blank scanline, and ends 4224 master cycles later. Register $4212 bit 0 is set during this time. Specifically, it begins at dot 74.5 on the first frame, and thereafter some multiple of 256 cycles after the start of the previous read that falls within the observed range.

Reading $4218-f during this time will read back incorrect values. The only reliable value is that no buttons pressed will return 0 (however, if buttons are pressed 0 could still be returned incorrectly). Presumably reading $4016/7 or writing $4016 during this time will also screw things up.

Registers

$4210 bit 7: shows the status of the internal timer's NMI output (1=low). Reading this bit causes the NMI output to go high.

$4211 bit 7: shows the status of the internal timer's IRQ output (1=low) OR the status of the CPU's external /IRQ line. Reading this bit causes the internal timer's IRQ output to go high.

$4212 bit 0: set when Auto Joypad Read is being performed.

$4212 bit 6: indicates H-blank. Set at H=274 of every scanline, and cleared at H=1.

$4212 bit 7: indicates V-blank. Set at H=0 at the beginning of V-Blank, and cleared at H=0 V=0.

$4214-7: 8 cycles (8 master cycles or somewhere between 48 and 64 master cycles?) after $4203 is written, the multiplication result may be read. 16 master cycles (or somewhere between 96 and 128 master cycles) after $4206 is written, the division result may be read. What happens before the time is up (or if you initiate another multiplication/division while calculating) is unknown.

S-PPU

The PPU is clocked off the same oscillator as the S-CPU.

Rendering

The PPU uses the same measure of frames and scanlines as the 5A22 S-CPU. There are always 340 dots ('pixels') per scanline; normally dots 323 and 327 are 6 master cycles instead of 4.

Note that a superscope pointing at pixel (X, Y) on the screen will latch approximately dot X+40 on scanline Y+1.

The PPU outputs one pixel every 4 master cycles, for dots 22-277(?) on scanlines 1-224 or 1-239. Note that (for NTSC) the color carrier is 6 master cycles, hence the interesting color effects seen with alternating pixel patterns.

The PPU seems to access memory 2-3 tiles ahead of the pixel output. At least, when we disable Force Blank mid-scanline, there is garbage for about 16-24 pixels.

Blanking Periods

V-Blank begins on scanline $E1 or $F0, at H=0, depending on bit 2 of $2133. If this bit is set, then cleared between $E0 and $F0, most "at the start of V-Blank" events will trigger at the next appropriate H point. Note though that clearing it too late may cause the TV to lose sync, possibly because the PPU doesn't get a chance to output a correct V-Blank signal. Setting the bit 'too late' will not resume HDMA or rendering or re-trigger NMI, but VRAM will be locked as if rendering were still proceeding.

V-Blank ends at V=0 H=0.

H-Blank begins at H=274 of every scanline, and ends at H=1.

Registers

$2134-6: Some unknown number of master cycles after $211c is set, the product may be read from these registers. What value may be read before these cycles have elapsed is unknown.

$2137: Read to latch the PPU dot position.

$213F bit 6: Set when the PPU dot counter is latched (or shortly thereafter), and cleared on read (and maybe the beginning/end of V-Blank too?).

$213F bit 7: Toggles every frame, at V=0 H=1.

OAM Reset

At the beginning of V-Blank if force-blank is disabled, the internal OAM address is reset to the value last written to $2102-3. byuu reports this occurs at H=10 on scanline 225/240. Also, byuu reports the reset occurs on any 1->0 transition of $2100 bit 7.

DETAILED RENDERER TIMING

Most of this is conjecture, based on a NES timing expetiment conducted by Brad Taylor (big_time_software@hotmail.com) around Sept 25, 2000.

All SNES VRAM memory access cycles are 4 master cycles long (the same amount of time it takes to output one pixel), compared to 8 (2 pixels) for the NES. The scanline is thus 340 memory accesses long. Also, the SNES PPU can access 2 bytes at a time (one from each VRAM chip) where the NES could only access one. Like the NES, 'rendering' begins on scanline 0, however nothing is actually output for scanline 0.

Beginning when the PPU begins outputting the first pixel on the scanline (just after H-Blank), we load the data for 32 tiles. For Modes 0-4, each BG takes 1 memory access for the tilemap word, and either 1, 2, or 4 accesses for 8 pixels of character data (depending on if the BG is 2, 4, or 8 bits, and see now why the bitplanes are stored the way they are?). For Modes 5 and 6, 2 or 4 memory accesses (again, if the BG is 2 or 4 bits) are required for 16 half-pixels of character data. Since this is at most 8 memory accesses for any BG mode, and 8 pixels are loaded at a time, you can see we just break even. For Mode 7, a tilemap entry is read from the low-byte VRAM chip and a pixel from the high-byte chip. During the rendering of the first tile, the third tile is being loaded from VRAM. This is a total of 256 memory access cycles.

Also during this time, OAM is being examined to determine the first 32 sprites on the next scanline. This (and the later loading during H-Blank) is the reason for the dummy scanline 0, otherwise there would be no sprite data for the first scanline on the screen.

During H-Blank, 68 memory access cycles are devoted to loading the next scanline from 34 4-bit sprite tiles, and 16 memory access cycles to loading the scanline for the first two tiles of the next scanline (recall that the third tile is being loaded while the first is being rendered). This totals 340 memory access cycles.

S-APU

SPC700

The SPC700 is nominally clocked at 1024000 Hz, however my SNES seems to run at ~1026900 Hz instead.

Instruction timing is not well known. There are some docs available, but their accuracy is suspect.

The SPC700 has 3 timers, one clocked at "64000 Hz" and two at "8000 Hz". In reality, the fast timer ticks every 16 cycles and the slow every 128, whatever length the SPC700 cycles really are.

The SPC700 communicates with the S-CPU via 4 registers. Exact memory access timings on these registers is not known, however it is possible that the 5A22 will be performing a read at the instant the SPC700 is performing a write. The 5A22 will then read the logical OR of the old and new values of the register.

DSP

The DSP outputs samples at "32000 Hz", although a real rate of one sample every 32 SPC700 cycles is more likely.

Other timing is not known. If you do know, please fill in the details here.

by Anomie (anomie@users.sourceforge.net)