Well it's too late but it seems you could have found your answer in the structure AD_SGL_WORK in SGL_WORK.h
...
#define Resolution __SGL_WORK->OPTION.Resolution
#define NbPCMBf __SGL_WORK->OPTION.NbPCMBf
#define PCMBufFlag __SGL_WORK->OPTION.PCMBufFlag
#define TrayCheckFlag __SGL_WORK->OPTION.TrayCheckFlag
#define SoundCount __SGL_WORK->OPTION.SoundCount
#define FRT_Count __SGL_WORK->OPTION.FRT_Count
#define SCUMC_ID __SGL_WORK->OPTION.SCUMC_ID
#define DMASt_CPU0 __SGL_WORK->OPTION.DMASt_CPU0
#define DMASt_CPU1 __SGL_WORK->OPTION.DMASt_CPU1
#define DMASt_SCU0 __SGL_WORK->OPTION.DMASt_SCU0
#define DMASt_SCU1 __SGL_WORK->OPTION.DMASt_SCU1
#define DMASt_SCU2 __SGL_WORK->OPTION.DMASt_SCU2
...
or in memory.txt :
060FFC00:
System variable area (GBR register always points to here)
0x400 bytes
System variables are as follows. They can be read from a C program
with the same names (identifiers).
* System Variables:
000: (EventTop) (EVENT *) ; event registered at top
004: (EventLast) (EVENT *) ; event registered last
008: (EventNow) (EVENT *) ; event executing now
00C: (EventCount) (Uint16) ; event remaining
00E: (WorkCount) (Uint16) ; work remaining
010: (MainMode) (Uint8) ; main sequence mode
011: (SubMode) (Uint8) ; subsequence mode
012: (SynchConst) (Sint8) ; video sync count
013: (SynchCount) (Sint8) ; video sync count
014: (UserFunction) (void (*)()) ; user function executed during V-blank
018: (TransCount) (Uint16) ; transfer count during V-blank
01A: (TransRequest) (Uint8) ; transfer request during V-blank
01B: (PauseFlag) (Uint8) ; pause flag for sprites and scroll
01C: (mtptr) (MATRIX *) ; current matrix pointer
020: (MatrixCount) (Uint8) ; matrix nest count
021: (PrintColor) (Uint8) ; color palette for text display
022: (IntCount) (Uint16) ; interrupt count
024: (MsPbufPtr) (Uint32 *) ; vertex coordinate calculation buffer
pointer (Master)
028: (SlPbufPtr) (Uint32 *) ; vertex coordinate calculation buffer
pointer (Slave)
02C: (SpritePtr) (Uint16 *) ; sprite data transfer pointer
030: (MsSdataPtr) (Uint16 *) ; sprite data set pointer (Master)
034: (SlSdataPtr) (Uint16 *) ; sprite data set pointer (Slave)
038: (ZbufPtr) (void **) ; Z buffer pointer
03C: (FormTbl) (TEXTURE *) ; texture data table
040: (SprbufBias) (Uint32) ; sprite data buffer switch
044: (ComRdPtr) (Uint32 *) ; command read pointer
048: (ComWrPtr) (Uint32 *) ; command set pointer
04C: (MsLightVector) (VECTOR) ; light source vector (Master)
058: (SlLightVector) (VECTOR) ; light source vector (Slave)
064: (ColorOffset) (Uint8 *) ; color offset table pointer
068: (MsScreenDist) (FIXED) ; screen position (Master)
06C: (SlScreenDist) (FIXED) ; screen position (Slave)
070: (MsZlimit) (Sint16) ; display limit Z position (Master)
072: (WindowNumber) (Uint8) ; number of windows used
073: (WinUseFlag) (Uint8) ; window use flag
074: (TotalPolygons) (Uint16) ; total polygons calculated
076: (TotalVertices) (Uint16) ; total vertices calculated
078: (MsScreenLeft) (Sint16) ; screen left position (Master)
07A: (MsScreenTop) (Sint16) ; screen top position (Master)
07C: (MsScreenRight) (Sint16) ; screen right position (Master)
07E: (MsScreenBottom) (Sint16) ; screen bottom position (Master)
080: (MsScreenSizeX) (Uint16) ; screen size X (Master)
082: (MsScreenSizeY) (Uint16) ; screen size Y (Master)
084: (MsWindowSizeX) (Uint16) ; window size (Master)
086: (MsWindowSizeY) (Uint16) ; window size (Master)
088: (MXPolygons) (Uint16) ; maximum polygons
08A: (MXVertices) (Uint16) ; maximum vertices
08C: (FrameSizeX) (Uint16) ; frame buffer size
08E: (FrameSizeY) (Uint16) ; frame buffer size
090: (MsWinXAdder) (Uint16) ; window check adder data (Master)
092: (MsWinYAdder) (Uint16) ; window check adder data (Master)
094: (SlWinXAdder) (Uint16) ; window check adder data (Slave)
096: (SlWinYAdder) (Uint16) ; window check adder data (Slave)
098: (MsClipXAdder) (Sint16) ; clipping adder data (Master)
09A: (MsClipYAdder) (Sint16) ; clipping adder data (Master)
09C: (SlClipXAdder) (Sint16) ; clipping adder data (Slave)
09E: (SlClipYAdder) (Sint16) ; clipping adder data (Slave)
0A0: (SlZlimit) (Sint16) ; display limit Z position (Slave)
0A2: (WinPtr) (Uint16) ; window data set offset
0A4: (DispPolygons) (Uint16) ; number of displayed polygons
0A6: (DMAEndFlag) (Uint8) ; DMA end flag (not used)
0A8: (DMASetFlag) (Uint8) ; DMA table set flag
0AA: (PutCount) (Uint16) ; slPutPolygon(),sl...Sprite() call count
0AC: (MsZdpsftcnt) (Uint8) ; screen display limit shift counter (Master)
0AD: (SlZdpsftcnt) (Uint8) ; screen display limit shift counter (Slave)
0B0: (Resolution) (Uint8) ; screen mode
0B1: (NbPCMBf) (Uint8) ; number of PCM buffers (2000H units)
0B2: (PCMBufFlag) (Sint16) ; PCM buffer empty status flag
0B4: (SoundRdCount) (Uint8) ; sound buffer read counter
0B5: (SoundWrCount) (Uint8) ; sound buffer entry counter
0B6: (FRT_Count) (Sint16) ; FRT counter
0B7: (SCUMC_ID) (Uint8) ; SCU memory copy channel number
0B8: (DMASt_CPU0) (Uint8) ; CPU DMA status (CH0)
0BA: (DMASt_CPU1) (Uint8) ; CPU DMA status (CH1)
0BB: (DMASt_SCU0) (Uint8) ; SCU DMA status (CH0)
0BC: (DMASt_SCU1) (Uint8) ; SCU DMA status (CH1)
0BD: (DMASt_SCU2) (Uint8) ; SCU DMA status (CH2)
0BE: (--------) (Uint16) ; system reserved
0C0: (VDP2_TVMD) (Uint16) ; TV screen mode
0C2: (VDP2_EXTEN) (Uint16) ; external signal enable
0C4: (VDP2_TVSTAT) (Uint16) ; screen status
0C6: (VDP2_VRSIZE) (Uint16) ; VRAM size
0C8: (VDP2_HCNT) (Uint16) ; H counter
0CA: (VDP2_VCNT) (Uint16) ; V counter
0CE: (VDP2_RAMCTL) (Uint16) ; RAM control
0D0: (VDP2_CYCA0L) (Uint16) ; VRAM cycle pattern (bank A0, T0_3)
0D2: (VDP2_CYCA0U) (Uint16) ; VRAM cycle pattern (bank A0, T4_7)
0D4: (VDP2_CYCA1L) (Uint16) ; VRAM cycle pattern (bank A1, T0_3)
0D6: (VDP2_CYCA1U) (Uint16) ; VRAM cycle pattern (bank A1, T4_7)
0D8: (VDP2_CYCB0L) (Uint16) ; VRAM cycle pattern (bank B0, T0_3)
0DA: (VDP2_CYCB0U) (Uint16) ; VRAM cycle pattern (bank B0, T4_7)
0DC: (VDP2_CYCB1L) (Uint16) ; VRAM cycle pattern (bank B1, T0_3)
0DE: (VDP2_CYCB1U) (Uint16) ; VRAM cycle pattern (bank B1, T4_7)
0E0: (VDP2_BGON) (Uint16) ; enable screen display
0E2: (VDP2_MZCTL) (Uint16) ; mosaic control
0E4: (VDP2_SFSEL) (Uint16) ; select special function code
0E6: (VDP2_SFCODE) (Uint16) ; special function code
0E8: (VDP2_CHCTLA) (Uint16) ; character control (NBG0, NBG1)
0EA: (VDP2_CHCTLB) (Uint16) ; character control (NBG2, NBG3, RBG0)
0EC: (VDP2_BMPNA) (Uint16) ; bit map palette number (NBG0, 1)
0EE: (VDP2_BMPNB) (Uint16) ; bit map palette number (RBG0)
0F0: (VDP2_PNCN0) (Uint16) ; pattern name control (NBG0)
0F2: (VDP2_PNCN1) (Uint16) ; pattern name control (NBG1)
0F4: (VDP2_PNCN2) (Uint16) ; pattern name control (NBG2)
0F6: (VDP2_PNCN3) (Uint16) ; pattern name control (NBG3)
0F8: (VDP2_PNCR) (Uint16) ; pattern name control (RBG0)
0FA: (VDP2_PLSZ) (Uint16) ; plane size
0FC: (VDP2_MPOFN) (Uint16) ; map offset (NBG0_3)
0FE: (VDP2_MPOFR) (Uint16) ; map offset (rotation parameters A, B)
100: (VDP2_MPABN0) (Uint16) ; map (NBG0 plane A, B)
102: (VDP2_MPCDN0) (Uint16) ; map (NBGO plane C, D)
104: (VDP2_MPABN1) (Uint16) ; map (NBG1 plane A, B)
106: (VDP2_MPCDN1) (Uint16) ; map (NBG1 plane C, D)
108: (VDP2_MPABN2) (Uint16) ; map (NBG2 plane A, B)
10A: (VDP2_MPCDN2) (Uint16) ; map (NBG2 plane C, D)
10C: (VDP2_MPABN3) (Uint16) ; map (NBG3 plane A, B)
10E: (VDP2_MPCDN3) (Uint16) ; map (NBG3 plane C, D)